Random crashes

For the longest time i've stuck with 3.95 because of this, but with all the new updates i figured i should give 5.0 another go. Essentially, i get these intermittent freezes of the game, i cant tab out, if i window the game i see the whole computer is frozen, 10 seconds later it seamlessly starts working again but i've missed like 10 arrows. These happen every 5 mins or so, sometimes more frequently.
plz help?

my guess would be a videocard issue, but is this happening only on stepmania, what happens if you run a benchmark?

i'm using a nividia quadro 4000 which really isn't suitable for gaming, but i've used it for cod 4 and counter strike source and have never had this issue. This may be the time to mention...
it idles at 85oC
it hits 90oC when stepmaniaing
all with under clocking o.o (by msi)
i have a small hell under my desk basically, and im just gonna burn it out till i can get a new graphics card
so im kinda scared to benchmark

it's definitely overheating, is vsync on? vsync will help keep things cooler.