Why isn't more likely to be something like bad capacitors on the inverter for the LCD display? For external monitors this is a very common symptom - I have a monitor on the bench that pretty much runs for a minute or two then the backlight goes black (but you can still the LCD pixels being driven if you look hard)

What impedances do graphics cards expect on H-Sync and V-Sync for VGA monitors?
imagine you want to use "should have done" in a subordinate clause when there's some past tense in the main clause.

e.g.: "He said I should have(had?) done it."

which one is correct? 
or should i use some other modal verb for this purpose?
thanks
