I bought a new Spider III 75W 1x12" amp about 5-6 years ago. I've only used it at home, so it's always been in my living room (no trips, no gigs, etc.) and it hasn't been used so much: let's say an average of 2-3h per week since I bought it (which is not a lot). With all these info I mean that it's yet "practically" new.
About 5 years ago I bought a new FBV Express footswitch, which is always connected to the amp (I don't think this is useful info anyway...)
The problem started about one year ago, but it hasn't become a real problem until last month (to tell the truth, I've used it very little during last year). While I'm playing, suddenly power goes off (you realize because amp lights and sound turn off) for short periods of time (not longer than 2-4 seconds). When it first happened, it only occured once and then you might not experience it again for weeks. But for the last month it happens periodically: you start playing and the issue may take several minutes (anyway not more than 15-20 minutes) to appear, but once it starts, it happens time after time with a variable delay of 10-30 seconds.
It has nothing to do with the footswitch, the guitar or the cables, because I've checked it happens even after powering the amp with all of them disconnected (in this case I know it's happening because of the lights going off). The behaviour is always similar: after some minutes, the issues starts happening periodically.
Any suggestions why this might be happening?