Holding note and tone just sort of "clicks" downward in volume

[tradarama]

I have a cool amp that sounds great. When I'm holding a note on the upper registers it starts out with power and thickness...after about 3 seconds it just almost audibly clicks and then jumps down in volume. The amp does use a variac in it; however when I run it at different levels I have the same problem.

Is it a tube issue?

I'm an idiot w/ these things.

Jim

[Troser]

I'm not a super-techie, but maybe a capacitor that is discharging in a non-linear fashion, in steps?