Did you recap the AB? There are known leakers in the video amplifier section. Also the pots need cleaned/worked back and forth. Also, did you check the solder joints on the CRT Neck board? You have to remove the shields to do so. If those go bad, youll get this problem. 

ALL, and i mean ALL EVERY SINGLE ONE of them capacitors have to be changed out in the AB. Not just a little handful quick fix shotgun thing. I mean ALL OF THEM.... Mainly the ones hiding in the shielded video amplifier circuit.