Could be heat related can you monitor temp of cpu / case?.
Other causes are full hard drive (no room for swap file), swapfile (virtual memory set too low), bad sectors on hard drive, faulty device drivers, spyware using all resources.
Hard drive full? you have done a clean install so maybe not that. Clear out all temporary files and internet cache anyway. Temporary File Cleaners:-
Crap Cleaner click here
Empty temp folders click here
Virtual memory increase if possible.
Bad sectors check harddrive. Hard disk tools :-
Seatools click here
WD Data lifeguard click here
Sounds like youv'e already re installed drivers.
Spyware? Anti Spyware :-
Adaware click here
Spybot S&D click here
Spywareblaster click here
a2 click here