buddy i have seen ur configurations.
U are having two HD 5870.So it should not happen.Normally this happens when u have not a good GPU.But ur case is contrary to this.
Buddy this can be ur display drivers problem.Try re-installing them.
The computer I'm using to type this post has an HD 5750 (1GB)
My other rig has not been assembled yet, and I still need to buy a monitor.
Off-topic, would X2 HD 5870's with 1GB of RAM each equal to 2GB effective texture memory? Just as Two 2GB cards would be 4GB total?