Bad Tube In My '66 Admiral ?

[ihmeyers]

I fear a tube is going bad in the recently set-up (with the help of the board) Admiral.

Shortly after I turn it on, maybe 2-3 minutes it the video flickers, not jumps but kind of flickers. I'm not doing a great job of describing it but it would be analagous to the noise a gassy audio output tube makes, almost like a sputtering. It will do that for a few seconds and then settle down, but might do it again another 2-3 times in a half hour period. What is (are) the likely suspects? I'm hoping its simply a tube going bad but without knowing which one it is there isn't much I can do other than replace a bunch of them. My wife made me discard my old tube tester a while back (no room).

Any help greatly appreciated!