chequered blue screen flashing

Yesterday I returned to the MacBook to find it flashing with a blue chequerboard effect (W about 4xH) and unresponsive. On reboot, the pattern showed through the apple and boot-up stalled after 1/2 min. Ditto from system CD & external drive.

After a bit of search (using another MacBook)the view seemed to be that it was the graphics card, but might be caused by overheat. This morning I tried a Hardware test from the original DVD which ran for about an hour/95% complete on the bar then stopped showing no fault detected. I did a bit of external vacuuming and then the computer worked normally uptill about 6pm today then went into the chequered mode again.

I would like to do a more thorough clean out and fan test before writing off the graphics card, but can find no info on this......Just open up and vacuum again and hand spin fan (actually is there one & where is it meant to vent out of) ? (using USB mini-vacuum designed for cameras). Assume it will need opening up to replace graphics card anyway.

Any further wisdom anyone please