The only pedal used at the time was a wah which was off and it is true bypass. The amp still has the original tubes in it. I bought the amp new in Nov. '05 and generally use it 3-4 hours a week.
Someone suggested checking for consistent line voltage. What do you think of that?
I've never noticed this with mine, but then I'm rarely pushing the envelope volume-wise. But I don't think it's normal. Check everything in your signal path including cables before you blame the amp. Also, when it happens, I'd completely remove the effects loop...unplug the cables from the plugs and see if the issue remains.