ok, my dad cleaned it, but that didn't really do anything. i measured the temperature from the GPU while shogun 2 was on and when it was off. when nothing was on, it usually was 70 ºC. when i started shogun 2, it started increasing to 90 ºC. when i was resuming as host for an MP campaign, it increased to 99 ºC! the fans were working at 96% of their power. when we were in the campaign, it immediatly dropped to 90 ºC, to eventually stabalize at 92-94 ºC. this was all on the lowest setting of everything, except for the fleet and unit size.

i decided to buy a new computer solely for gaming (see this thread). i still don't know why it happened. i will play on the lowest settings from now on, untill i have a new computer.