Does it seem to affect all frequencies equally, or is it focused more towards the bass/high notes? Is it only when you play loud, or is it at any volume?
Thinking of things that it's not likely to be, we can pretty much rule out the trannys (they did a "how it's made" segment a while back... they use asphalt to hold the core in place...), and it doesn't sound mechanical (otherwise it would sound like crackling or cutting in and out).
Since these do not have a power switch, turning it off may be draining the filtering caps and turning it back on may charge them up briefly, only to drain again. If I had to venture a guess, you are probably using the original caps, and their time may have come. Fortunately, they are easy to test with a voltmeter.
The other thing I've heard happening with the old Macs is that the transistors may eventually fail, resulting in "ringing". In the 250, this typically manifests itself as one channel sounding bad, but you've got yourself a mono amp, so the whole thing would sound off. Unfortunately, this is real hard to diagnose without an O-scope.
"Learn as if you were to live forever; live as if you were to die tomorrow."
