[noodles]

The shop you took it too sucks ass. Your description sounds like a classic power caps problem. Does it go away when you roll back the volume on the guitar? Does it have mechanical artifacts at loud volumes (sounds almost like Neo in the Matrix when he takes the pill and his scream turns mechanical)? I bet it is still there on the dirty channel, but you just can't really hear it because the distortion hides it.

I had the same problem on a Line6. I took it to the shop, and it was better for a few weeks, then started again. The shop only replaced one of the caps, and the other one finished going out a few weeks later. Sounds like your shop took the same cheapo route.