Make sure the 12au7 reverb driver tube is in good shape; if you're in doubt it won't cost too much to replace it...AND...check to see if it is an actual 12au7...sometimes a 12ax7 has been mistakenly used as a replacement, and that tube of course has a higher gain that will overdrive the reverb tank. Take the tank out to check if anything is touching the reverb springs, and if the send/receive cables are OK, use a contact cleaner spray like Deoxit D4 to clean the RCA jacks.