One of my 5870s started getting slower and slower when crunching DNETC due to the VRM getting too hot. I had to keep downclocking it until finally got down to stock speeds. Finally even at stock speed if I hit it with DNETC it would instantly cause a failure and lock the computer requiring a hard reset.
I sent 2 cards in for repair, this one and one that had a fan failure. While I was on vacation I got an email saying one card had been replaced with a new one and that they were unable to duplicate the failure on the 2nd. I assumed the fan had started working again.
Wrong, the card with the bad fan had been replaced with a new card and the one that wouldn't run DNETC was the one they couldn't find a problem with.
I called XFX Technical Support and explained what had happened. The guy was familar with both Folding and BOINC and when I explained the failure mode and that it was high temperature on VRM1 he conceded there was a problem with the card.
XFX is sending me a new card.
Folks this is what I call customer support! The purpose of this post is just to give a data point when considering which GPU vendor to purchase your cards from.