SC2 freezes after 5 mins since having played "phantom mode"

hi all,

I know I'm not the only one who's sc is freezing/crashing but I have noticed that my case is a bit different from what I've seen so far.

The first 1/2 months of playing starcraft 2 worked perfectly fine, never had even the smallest of lags. That's probably because I've got a great clean pc. I'm not at home right now but I'll post the dxdiag when I get home (it's an i7 quad core, GTX 260 etc.).

BUT: when I tried to play a game mode called "phantom" my pc suddenly froze to a complete stop, no pixel streaks or blue screen of anything else, just a frozen screen with the sound repeating all the time. Ctrl+alt+del did'nt work and a hard reboot was the only solution to get out of it.

Since that one phantom game my pc freezes a lot (>50% of the times I play a game) and sc2 has become nearly unplayable.

Weird thing is that it always seems to happen around 5/6 minutes into the game.

I have once tried to play the single player campaign, this went well for more than an hour, but after an hour and a quarter it crashed/froze anyway.

I don't think it's an overheating problem because I've got a nice airflow with (at least) 2 fans, and above all: have never had any other problems with whatever other game, AND sc2 worked fine for the first 1 or 2 months I played it.

I updated my drivers yesterday and that didnt help either .

do you guys know what the problem might be?

thanks