Hey Tom,
I am having similar issues on my 55'. Warm up is slow. By that I mean from a cold dead start, convergence is WAY out, by about 1/2". As the voltages stabilize, convergence comes in very nicely. It takes a good 15 minutes from cold. Shut it down for a minute and you are back to way out again, but will come back into coinvergence faster if the set has been previously warmed up.
My linearity is very good, and HV regulates nicely at the 25KV spec. I too am running on a variac, at around 110vac, which gives me a spot on 400vdc B+ If I run off of 120 line voltage I get 437 vdc B+ and my flyback runs 20 degrees hotter. I already toasted the origanal, so I am being most carefull to not destroy this one. Running at 400vdc the flyback operates at about 125 degrees F after several hours of run time and also with the dog house panels removed. After I have the thing running to my satisfaction, I will reinstall the dog house panels and install a muffin fan to keep the dog house internals cool.
I would be interested in knowing what your flyback temp is after you run for several hours, just for comparison sake. I use a DMM with a thermister probe to take temp readings. I of course do it with the set turned off and the door knobs discharged. I place the probe on the surface of the primary windings and hold it there till the temp reading stabilizes. I like 125 degrees F. It seems to me to be a safe operating temp.
Been trying to run down an intermitant flashing brightness issue, but haven't had much time to trouble shoot lately. I am pretty sure if I stick at it long enough I will locate the gremlins. that is, if I live long enough :-)
I like Mark's suggestion to try freeze spray on resistors to see if I can cause the convergence to drift out. Maybe I can find a resistor that is overheating and changing value as the set warms up. Being as convergence is done with the set warmed up, it would be logical that when the set cools down it would be out of convergence if a resistor is changing value as it heat up. I just ordered up 3 cans of freeze spray. I found it online for only 4.99/can at
http://www.southernhillscomputer.com/index.html some places wanted $10 per can. The stuff is R134a refrigerant. I am epa licensed to do refrigeration repair and buy this stuff in 30 pound drums normally for repairing air conditioning systems, but 30 pound drums are a bit to large for squirting resistors with :-)
Bob