[codefox1]

Agree with the Master Paul, a good scouring of pins and firming up contacts of sockets using Deoxit and an old toothbrush or more drastic approaches like brillo on the tube pins, (rinse and dry carefully for a day,) would rule much of this out. Not a bad thing to do no matter what.

"Thunder" in an otherwise properly working set after connections, faulty caps and tubes are ruled out, is almost always a bad mica cap, usually one in an IF can that is a real pain to et to and replace, and can not be measured by any method published so far as I know. Of course you could isolate the problem by stage (IF,RF,) you probably know the drill, but as said, best of luck.