I'm experienced with electronics but don't know much about TV's. Can anyone
help me identify this problem?

Panasonic TX-W28R4
Recently: After switching on from cold every now and then the picture starts
to shrink in the vertical direction. I bang the top a few times over a space
of 15 minutes each time this happens after which it's warmed up and works
fine. Sometimes the picture shrinks to the bottom inch or two of the screen
and is scrambled. Occasionally the screen goes blank and the TV shortly
afterwards shuts itself off.

I lived with that but now the problem is worse. A few seconds after
switching on and nothing appearing to happen, there is a flash on the screen
of light collapsing towards the centre as the TV shuts off. I can't use it
at all any more.

I vacuumed the circuit boards free of dust and this gave me two more days.
I just tried pulling/reinserted all the connectors but that hasn't helped.

Any ideas gratefully received.

Scrim