Hi Blusyko
I have a very similar problem, an low an behold im also usin media centre, lol
Now i think ive got it down to one of two things
1. drivers....i just read something about using 66.xx version drivers, possible i suppose!!
2. Psu rating, unfortunatly i have the card in my shuttle pc which has a physically small psu and has no power descriptions on it whatsoever
BUT!!! i have found a temorary fix, get coolbits, and drop both the clocks by about 50mhz on both settings, this appears to give the card a little more stability,
May be worth looking for a newer bios for the card but i got a little more investigating to do yet
Hope this helps, keep in touch as our problems seem a little too similar to be coincidence
Chris