[35Z5]

There will be a major howl, moan, groan, etc from the gallery but after popping starts, try this.

Look at top of IF cans, note slot position of screw. Adj counter clockwise approx ¼ turn any of your choice. If it makes no difference, return to orig position and go to next adjustment.

I'm thinking you may have arcing under one of the mica wafers inside can. Anyway it's a far more likely sereno than a bad tube socket.