I have a dual 5970 mining rig, and I use CGminer. I use 800mhz core and 267mhz memory. I don't know what on earth could be the problem... My GPU core temps don't go over 80c and VRMs don't go past 110c.
When I witness the crashing, I notice that the ATI driver goes first, and then CGminer crashes.
Try it at stock clocks to rule that out entirely.