I've got a classic 30, recently replaced all the power tubes hoping to solve this problem but it didn't.

The normal channel works fine, but the pre gain channel moves in and out.

Sometimes it sounds fine. Other times it gets really quiet and tinny no matter how high the post gain is turned up. Sometimes it even mixes the two and is loud but lacking pregain.

When its acting up sometimes I can make it(temporarily) work by bumping my fist on a certain spot on the top of the amp.

Has anybody else experienced similar problems? What might it be?