George,
Many thanks for emailing the manual. I've managed to fix the problem
but I'd like to go back over it again once I get to figure out how to
get the set into Service Mode (more on that in a moment).

Section 4-4 of the manual (SCREEN Adjustment) was the clue I needed. At
the foot of the page it says "If you do not follow this procedure, the
picture may be abnormal. For example, when the TV set is on, the
picture becames(sic) brighter gradually". This tied in with the fault
disappearing when the colour temperature was changed and also by it
being ok briefly after switch on (on a warm set) - something had
drifted and the set was compensating.

I managed to correct the problem by *slightly* adjusting the Screen G2
control clockwise [this is the bottom of the two controls on the line
output transformer].

I hope you have the same luck as I did. If you find that the Screen
control on your set needs a large adjustment then that might indicate
another problem. Let me know how you get on.


Now then, SERVICE MODE...
The instructions in section 4-7 of the manual didn't work for me,
namely:
[PICTURE OFF]-[DISPLAY]-[MENU]-[MUTE]-[PICTURE ON]

Quite possibly I'm not translating the instructions into the correct
buttons on the remote. I also tried Skype_man's instructions (above)
and they didn't appear to work either. Did you manage to get into
Service Mode?