Starting with the obvious . . . I assume you have already done the usual swapping of horizontal tubes with known-good ones to eliminate those as an issue? Especially the horizontal oscillator. A tube tester that checks a tube's emission can't necessarily test how it will function as an oscillator. So, a tube might test way up into the "GOOD" zone, yet still be a lousy oscillator.
Just a thought . . .
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html