augh the dreaded white screen of death...
Driver problem , dx corrupted ,power supply dieing, video card worn out or overheating ,and last but not to be dismissed is a possiability of bad ram.
try turning off all the extra fans and run a house fan blowing into it . if it runs ok then its most likely the power supply.
also check that All the fans turn easy , arnt draging ,and are working smoothly .