My g/f's 9500 was doing something very similar... sometimes THE SIMS would lock up on her and when closing manually windows would be at some stupid resolution.. like 320X200... A reboot fixed this but it happened about every 3rd or 4th time she played.
Shortly after, I upgraded my video card and gave her my 9800GTX which corrected the issues.
(I should also note that we put her old 9500 in her dad's computer and it completely failed a couple weeks later. After replacing it with a new card of the the same identical one, his computer was back up and running)
I'm banking on a hardware issue, but it could be a drivers thing too...