I have a 5870 running poclbm and after about 10 minutes of mining it drops from ~315 Mhash/s down to ~230 Mhash/s.

I'm running Windows 7 32-bit with dual 5870s, the 2nd 5870 seems to be unaffected by this problem and runs full speed all the time.

Anyone else encountered this issue?

Yeah, I thought my cards were fine temp wise but apparently these non reference 5870s have lower throttling temps.  I sorted it out with some fans and such.