Here's the field output stage:
The output chip, IC301, is type OIPRP00741A and I can't find any information on it.
I would suspect IC301 first. Let the set warm up and when the fault shows up, spray IC301's heatsink with aerosol rapid freeze for a few seconds and watch to see whether the fault goes away. If not, you could also try D302. Also watch the +26V rail on the anode of D302 to see if that changes when the fault is present.
Does the picture at the top of the screen curl over or get squashed when the fault is present? If not, it's also possible that the vertical output stage is OK but there is a problem with the blanking signal.
How much experience do you have working on TVs?