I had similar occurrences, also on a 5870 (and Windows XP). The most recent was that I left my miner (poclbm GUI) running, and when I checked it some hours later the hashrate was 140 MH/s. Restarting the OS restored the normal hashing rate of 315 MH/s. Any ideas about the cause of this?
Yeah this has also happened to me, however this time, the hashrate never restored itself to its previous level of performance.
This might be a case for a dedicated rig.
Nah not really worth the investment considering I only have one 5870, i might just turn off my miner or remove the overclock when I play games. Dunno tho.