This is still something that needs to be fixed with the game in the long run, this is... ridiculous. >_>; I appreciate you going through all the things you've tried!!
I tried most of these already without success (Only thing I haven't done is a clean install of windows and disabling *all* unneccesary services since I'm not tech savvy enough to be sure I won't botch things, hah).. For curiosities sake, how far did you underclock things? this is the first time I've had to underclock so maybe I didn't do it enough/right. I'm willing to reinstall Afterburner to try, plus I have a Nvidia thing I was going to try possibly as well to underclock.