Thanks so much to all who have replied and helped with this with the great ideas and suggestions!

After several days of testing with the proper power supply I have had only a single crash which was much easier to recover from. Though it acts very similarly with the slowing of the system until the offending WoW client is killed, it is not entirely unusable. I can live with an occasional crash.

The system remains stable with up to 24 hours straight of Prime95 and other stress testing. I've not had any errors so far in any such testing. Using HWinfo I see that the only error during this crash was CPU 0 DTS: Core #2 Power Limit Exceeded Yes. No other core showed this error. Possibly coincidental I have the CPU cores assigned to individual WoW clients in order. The third client is the one which crashed and that was assigned to CPU core 2, the third one.