OK thanks for both of your replies, helps a lot. its a new build so its not a dust issue however I stupidly used the pre applied thermal paste that came with the heat sink even though I had some Arctic silver 5 sitting next to me !
I Dont really want to reapply as I had trouble getting all 4 pins in as it was
Obviously I am over clocking the CPU so this would also increase the temp and it only happens with single cored games such as left 4 dead 2.
Also i think i have a solution to why I BSOD'ed. i was running avg doing a scan while playing crysis 2, after looking in the avg log it seems that it had picked up one of the crysis 2 dll files as a virus and automatically removed it thus causing my game to crash and cause a BSOD.