Definitely not a flyback issue.
Btw, in the other thread you posted RGB pictures that look fine.
Are all the pictures 50Hz or 60Hz, or are they mixed?
Its not clear from the description, since later on you write that PAL (50Hz) looks fine on composite.
As spaceman88 suggested, get a freeze spray, let the monitor warm up and start exploring.
I'd start with the PSU, then move to the HV section.
