SAMSUNG lcdtv al29a0 MODEL LW29AI3W?

My son bought this TV second hand with his hard earned paper round/birthday/Christmas/Odd job money... it now has an intermittent frustrating fault. If any one can throw light on this it would put a smile on his face...!

Power on - screen is black - All is well and TV works for a few days.

Power on - Screen is white - Any Video shows as odd green/purple blocky colours... this will happen every two-three days and by repeatedly turning off/cycle mains power, turning on (for around 30-60 minutes) eventually it will come on with a black screen and work OK again.

Sound is good all the time.

Same effect on AV or RF input...

Any thoughts or ideas that anyone out there can throw this way will be most appreciated by a frustrated Dad who cant get his kids toy fixed!!

PS. I have taken this to an Ex Radio Rentals TV repairman who had it for 9 days before proclaiming "Mmmm - It has a white screen but the sound is OK! I dunno how to fix it though!"