hi ive got a peavy  bandit transtube 112 the older version i bought it from the pawn shop and its a great amp, but i plug in turn it on and start playing all of a sudden the volume goes down then a minute later it goes back up? no matter what channel its on or volume settings, and when it goes down u can still hear my playing but its like some invisible gremlin is turning the volume down!! i need to know if this is something where i can get a replacement part and fix myself or spend the MONEY for a guitar shop to fix it. i hope i given enough info for someone to help cause i really like this amp, thanks for any info you can give me, help me to play again AMPLIFIEDDDDDD!!!!