I've only experienced the minimizing to desktop issue, that was resolved by getting rid of AVG 9, other than that, it runs perfectly on my system, and I have physics disabled since nothing really utilizes the feature.
Yeah it sounds like something is screwed up with your system, and yeah 20 minutes into a session sounds like a heat problem, damn good clean with compressed air in the GPU heatsink and fan, CPU fan could probably do with a clean as well, if that doesn't do it, then try re-seating memory and GPU, after that, I'd start looking at the operating system, maybe a graphics driver problem, or the OS is screwed up somehow.