4870 boot fail/corrupt looking display

Hi i built a computer about mid last year and installed Vista Ultimate...and its been working fine.

Recently i installed a new HDD (WD VelociRaptor 300GB) and installed 7 Ultimate...and it worked fine for about 2 days then once in a while just wouldnt pick up the gfx card during boot...which i wasnt sure why maybe just a loose monitor cable or something but after that didnt work i updated my cards drivers with the latest from ati and then when that didnt help got the latest from Asus cause its an EAH4870 by Asus.

Now still no improvements, only boots properly with display if i havent used it for hours and mostly doesnt boot with a display, keyboard or mouse (dont think it even goes into windows) and sometimes boots with a corrupt looking display with lines and/or blocks everywere and then just fails and restarts at the point of loading windows and sometimes give a BSOD something to do with an atidiag file...doesnt stay on long enough to read.

Sounds like its overheating hey?...but it worked fine before and i always use fan profiles to keep the temp low (when it was working atleast).

Please help!