@klataa -
I have single 5870
Are your runing oc'd?, try adjusting one at a time to find the culprit: vc core clock, vc mem clock, fan on full blast?, vc voltage - likely it is one of these.
Could be outside the video sybsystem but check VS first.
If you have everthing at stock on the vc's and you still have the problem then you need to look elsewhere - cpu oc? mem oc? sufficient powersupply?

I feel for ya - these things are very hard to diagnose.