Some cards have trouble going from full load to zero load. Perhaps it's overly-aggressive in underclocking when it's done at full load, or could be related to the PSU -- I'm not sure. Does the same thing happen if you underclock your card and then go from full load to stop?
However, I don't think it's necessarily the 5850 model (brand may, however, be an issue) or Win7 since I'm running Win7 with 5850s and don't have this problem, though I did have this problem occasionally when I was running an overclocked 460, and I have no idea why (it had no stability problems outside of this). Wasn't just mining -- starting/stopping Furmark would do it on occasion, too.