i get that, but ONLY on nfsmw. i think it has something to do with the dual thread capability of my processor... (HT). i read that to fix it, i should load the task manager once the game has loaded, and set the affinity of speed.exe to one virtual processor only.
this didnt actually help for me, and looking at your spec, you dont have dual core.
BUT, i think this is definatly a software issue, as i know two people who have this game, one on a 7800GTX, one on a 7800GS, i myself am on an X800GTO, so none of us are on poor cards. we all have this problem. are you guys saying that we all need to up voltages, just because we all get it on one particular game? hmmm...