Kemper is fine but sorry, I can't stand G string... really... enough.

Hi
It's usually the nut that's the culprit. Haven't needed it but "nut sauce" is supposed to be good to lubricate the spot.
if you bend the g string and it goes out of tune before using the tuning peg to retune try to bend the string behind the nut with your thumb then play the string again.. if its back on tune again the string is getting caught on the nut..

I once heard that there is actually some magic to it, though I can't remember the speculation around that topic. And the magic was that the g-string for some reason ALWAYS sounds like it's slightly out of tune (in this case you probably meant that it actually goes out of tune, but this thing just crossed my mind).

Would be nice to know why the string always sounds so ugly, imho it's particularly obvious when using slight distortion.

Firstly the caveat: I know nothing about guitars and can't play for peanuts (or bananas).

At first glance the G string looks to me as if it's the "compromise" string. It's probably a little too skinny to be wound as well as being thick enough that it "wants" to be.

IOW, the thin strings look at him and say, "You should be wound, G; you're practically obese", and the thick ones, wishing to wind him up, say, "You should be wound for sound, bro'".

Whatever the compromise entails, I'm betting that that's where the origins of the instability lie.

It seems your G string is binding at the nut slot. Putting oil (nut sauce) or using a thin graphite pencil to coat the nut slot will help with that.
Not very common in designs where the string goes straight to from nut to the tuner, but it seems to be your G string is a tad too thick to that nut slot which might be the reason why its binding.

I've heard that if you loosen the string sufficiently to be able to lift it out of the nut slot and drop it back in and it "sticks" repeatedly, the slot needs to be widened a little.

As Havohej implied, a slot which is too narrow will cause sticking and therefore the tuning instability you speak of, Skoz.

I have heard good things about Nut Sauce too, I haven't used it my self. I find a good setup guitar will make sure things stay in tune also

Another possible cause, as you say it happens on all of your guitars, is your method of winding the strings onto the tuning posts. There are different ways of "locking" the strings onto the posts so that they don't slip, but I believe it's quite important to have as few windings on the posts as possible.

Nut Sauce all the way!
I plaster my guitars in it. and never have any problems.. apart from the player of course : )

Always with the G string... Only my PRSs don't do this... Too much...

Big Bends Nut Sauce, or the brand I use but can't remember the name of.... EDIT: It's what lef38 suggested below - graphit-all guitar lube.

A relatively skilled technician can also file it just a tad to make it work better, probably even without going too deep or wide - sometimes

Quote from Lati

I once heard that there is actually some magic to it, though I can't remember the speculation around that topic. And the magic was that the g-string for some reason ALWAYS sounds like it's slightly out of tune (in this case you probably meant that it actually goes out of tune, but this thing just crossed my mind).

Would be nice to know why the string always sounds so ugly, imho it's particularly obvious when using slight distortion.

There is some truth to this, actually - equal temperament tuning, which most instruments use (apart from those crazy fanned fret instruments etc), is always some kind of compromise. The Buzz Feiten system seeks to alleviate this (though I'm unconvinced how well that works if the guitar is not the only instrument in the band, drums notwithstanding), by having the contact points of individual strings being slightly shifted from the "normal" type nut. And I do believe the biggest shift is on the G string.

However, I don't experience the alleged disharmony when playing myself

I'm successfully using this (same as nut sauce) http://www.thomann.de/fr/ghs_graphit_all_guitar_lube.htm

Also when i have string guides like yours, I immediately change them for this kind (or roller ones):
http://intl.fender.com/fr-FR/g…-string-guide-black-each/
Less contact with the string, gives less opportunities for it to hang up
(or at least put nut sauce there too !! )

I've had good luck with tusq nuts and string trees.

http://www.stewmac.com/Hardwar…ITZlumOmMsCFUk6gQod1woBtw

My PRS SE in my avatar caused me no end of annoyance regarding tuning stability, particularly on the D & G string. Like the Chapman you've got, both had straight string paths and both are made by World Guitars in Korea (unless yours is older and Chinese made). I replaced the tuners, refiled the nut and fiddled with the bridge to no avail. I then bought a Graphtec nut (<£10), replaced the old one and it has been rock solid ever since. The nut was a doddle to fit by the way.

The g and b strings are tuned to a major 3rd rather than perfect 4th. While equal temperament 4ths and 5ths are close to their just intonation counterparts, 3rds are quite a bit off. Maybe thats what you are hearing.

Interesting stuff, here's an article by an old friend of mine which discusses the physics of a G string and inevitable tuning problems.

http://www.edgeguitarservices.co.uk/scheitenballs/

General ideas:

01) Have new bone nut put on
02) Nut sauce as already said
03) Check string trees for binding, put sauce on those as well. (2 tees speak for a poorly angled neck in the guitar build IMO)
04) Get locking tuners. The best ones allow you to adjust the pole height so you can create an angle and do away with the string trees.
05) If you have a Tremolo and can live w/o it, inserting a block helps. Or buy guitars with through the body bridges. (better sustain too) I assume you bend a lot, so you probably don't need a tremolo going all Jeff Beck on the G-string)
06) I can see that you aren't winding enough on the poles. Try at least 3 wraps of the string on your poles.
07) Are you changing your strings enough? Dead strings get tight and bending pulls them out of tune. they can get this way sitting for a month, or played for a few hours in one night of lots of bending.
08).String type. Slinky's can bend easier than D'addario. But they also sound very different. Always a compromise.
09) You can play detuned 1/2 note. Less pull and therefore less detuning.
10) You can get a TronicalTuner for your guitar and tune it in a snap automatically.
11) Another factor is the scale of the fretboard. Smaller scales bend better w/o losing tune. My MusicMan can be bent all day long, no stress on the fingers, no detuning (well it also has locking tuners which definitely help) But it's not for people with big hands. I have medium sized hands and I often can pull a string off the board by accident getting a little crazy.
12) go all Zen and become One with your detuned tones
13) become a bending master and make your bends correct the tuning
14) stop playing guitar. as the Tao Te Ching says, "If one does nothing, one has nothing left to be done". so wise...
15) this one escapes me for now. thats what age does to ya. maybe it involves a V.I. guitar plugin and MIDI keyboard?

That's all I got for now.