RMA your PSU now and replace it.
I had a 750W PSU running 2x6870's for little over 12 hours before it literally went bang and sparks shot out of it. For those 12 hours it was a kinda "hot" smell, even though the PSU didn't feel hot. Faulty capacitors. Best be safe than sorry
I'm running 1x7950 + 1x7970 on 700w psu. Your psu was deffinetly faulty cause one 6870 power consumption is about 150w