Why wont my graphics cards work.. ??

Can someone please tell me how my graphics card drivers just gave up one day?

I had an ATI Radeon x300 card, worked fine until about a fortnight ago when the 3d acceleration driver packed up mid game,

upon a much needed hard restart it worked again for about half an hour.. the graphics completely crash at irregular intervals now usually but not exclusively whilst running 3D apps or flash based sites. Since the crash ive run all the diagnostics i can think of all the hardware is working fine, ive even tried another video card brand new it exibited the same problem, ive tried all possible drivers available from oldest to newest.

Direct x diag shows the driver to be working fine before a crash but reports a problem after a crash.

I've tried re-intalling Xp professional and im now quite stumped.. i just dont know how to solve this.

Im running 2gig of ram, a pentium 4 processor, and an Ati radeon x300 card (the replacement was also the same model)