Now that u mention it, I did encounter such situation a few times since my overclock. I think it has something to do with the overclocking process itself. Chips are made to run in binary system, that applies to their speed too. In other words, try to stay away from odd numbers when u overclock (eg 213mhz). Instead keep to 2,4,6,8 and 10 if possible (210mhz). Another probability may be overclocking the RAM itself. Not much info on that tho'. Anyway, my comp's seems to be quite stable now.
I've also tried blowing away the dust. But the odd thing is i use to run my GPU at a stable 57 degrees at rest on Vista. This problem only arises the moment I installed Win 7. I updated my BIOS and it seems to help a little but still could not achieve my previous temp. Maybe i'll try to search out for alternative open source drivers for my GPU