That is bizarre.
How old is the amp? Do you have a spare to try?
Preamplifier? I had a preamp do the same thing a year or so ago.
I would start from the speaker back. Try and get a spare amp to test. Maybe pick up a Crown from Amazon or Guitar Center to try. They are cheap, like 200-300. Might sound terrible, but will let you know the problem.
If it’s not the amp, work backwards.
Is the amp bias meter acting funny?
Do you have a tube preamp?? Could be bad tubes?