Better replace the Tubes..
..naa just a joke 
i heard such problems often from the magnum44 because if overheating
sound s like a Magnum issue to me, too.
I'll move the thread to Other Gear
I had the same issue with the Magnum 44. I seemed to occur when it was driven pretty hard. I returned it a got a ISP stealth instead.
I use Kemper eith Line6 relay wireless, never had a problem exactly the same sound and feel as with a cable...
Besides your issue I read several times, that the Magnum runs into distortion with a setting past noon. You don't want that for the Kemper...
I just realized I was going Straight to the Active Speakers... FR FR.. So the Magnum was not being used! 
What could it be? I had rehearsal today and it Brrrppptt again. Its just a brief disconnection, like half a second for 3 times... like TAK TAK TAK... and then comes back to life
What active speakers are you using? How loud are you? They may be overheating and shutting down.
I just realized I was going Straight to the Active Speakers... FR FR.. So the Magnum was not being used!
Difficult. I'd open a support ticket anyway.
perhaps off topic, but this happened to me.
And the problem came from my guitar with a wire unsoldered connected to my pickup selector, sometimes it made contact, and sometimes not !! Same problem with another guitar ?
