TLDR: System craps out when playing HD after ~30 minutes. Waiting 5 minutes fixes it.
I think I have finally determined my weird issues to be hardware (heat) related.
I'm using my mini for "heavy" video tasks, such as watching 720p mkv video or running slingbox software. I'm using CoreTemp to monitor CPU speed and temp and the task manager to monitor CPU usage. I idle at ~73C, 800MHz, 2%. When watching 720p material, it jumps pretty immediately to ~82C, 1600MHz, 50-80%. However, the temperature slowly creeps up. When it hits ~90C, my CPU usage pegs itself to 100% (still 1600MHz), the video starts slowing down (i.e., I get fewer FPS but it insists on showing all of them) while the audio continues at normal rate. This means I rapidly lose audio/visual sync and things become unwatchable.
If I seek back to an area before the bogging down, the issue persists. If I close media player and reopen it, the problem persists. If I try opening a less intensive video that would normally play fine, the problem persists. If I wait 5 minutes, the problem goes away until the temp creeps back up.
Most of this testing is done with media player classic/coreavc, but I see the same thing with pegged CPU when playing with my slingbox. I think the CPU usage is some sort of resultant error as a result of something (GPU?) becoming overheated rather than a direct cause of video problems, given that it doesn't seem to be a result of the media/scene I'm watching.
Has anyone else encountered this, or does anyone have any ideas on how to combat this?