How many pins do your PCI-E power headers have?

2x6pins are revision 1 cards running on the original 7950 boards (which apparently ATI never gave a reference for)
the 1x6 + 1x8 is the revision 2 card - it runs on a 7970 board.  That extra power might add to your stability.

I use a rev 2 and it hasn't ever failed (admitedly I have never pushed it past 1200mhz).

I also underclock my ram to 310mhz - running on server 2008 r2 sp1