Knock on wood (*knock* *knock*), but I think I might have it fixed now.
This afternoon, I switched on the set and right from the get-go, the HV just flashed and flashed non-stop. So, I put it on the "bench," pulled the back off, switched it on again, and wouldn't you know-- it ran perfectly. I must have let it run for an hour, and it still ran great. Tapping the cabinet, the chassis, and various parts didn't have any effect at all. Switched it off, came back a few minutes later, switched it back on again, and now it's in disco mode, flashing on and off like mad. Nothing seemed to consistently get the intermittent to start or stop. Tried switching it off, waiting several minutes, switching it back on-- sometimes (one time anyway) it was well-behaved, sometimes it ran a moment, flashed a few times, then worked great again, and sometimes the HV just cut off and on constantly.
Well, lacking any better/easier ideas at the time, I figured, eh, how about just trying a new 6GH8A horiz osc tube and see what happens (I knew I had a couple of spare 6GH8A's around). And wouldn't you know, but it looks like that was it after all. After swapping that tube, it's been running great, even after a few restarts and letting it run for 10-20 minutes. I guess that tube had just developed some sort of weird intermittent; even tapping the tube and wiggling it in its socket wouldn't trigger it, but I guess ya never know..!
Funny how it turned out to be such a simple fix though-- I shoulda thought to try that earlier..! ...chee...
BTW, bgadow, when I first got this set a few years ago, it had a case of bad vertical linearity, and it too needed a new mylar cap in the vertical osc circuit to make it run right.