Yeah – don’t do the softmod. While it works most of the time, sometimes the reason a part is a Pro versus an XT or some other distinction (such as between a Pentium 4 and a Celeron) is due to a defect on a portion of the die. When you activate that portion you get problems, and that’s what I’m 99% sure is happening in your case.
The other 1% is that it needs more cooling with the extra heat load, so try that too.
I know it’s not the best news, but it is kinda true …
This is why on my main computer everything runs stock.