Moderator, please move this to the Rectangular tube & Solid State forum.
Roger, you'll get alot more responses there
in any case, sounds as though you have a problem in the HV shutdown circuits. What does your HV probe show at the 2nd anode (HV lead)? If its more than the allowable voltage, the set will shut off. If its within factory spec (look at the service manual) then there is more than likely a faulty component in the HV shutdown circuits.
Sorry i can't be of any more help to you, but I'm more of a tube TV kinda guy.