Yeah, you aren't alone.
DF stops folding as evidenced by CPU = 0% in task manager. Unfortunately, the program stays resident in memory so the service is not identified as stopped and doesn't automatically re-start if you had configured XP service that way.
Re-Booting brings it back (sometimes takes 2 times)
For me, it seems that playing BF1942 (on-line) brings it on.
I suspect that any prolonged 'streaming' app. will cause problems.
Howard has indicated to me this "bug" won't be fixed except in the Beta client.
-Sid
edit: There's a couple threads about this in 'Bug Tracking' section