Disable ulps, or leave the second card@stock.
Trixx, Gpu-z can cause a CrossFire enabled rig to hang the system I have noticed in the last couple of days now that I have my DD cooled properly.
After you boot, you can launch Trixx fine, close it or minimise it or/and launch it again results in system hang needing a hard reset, Gpu-z is doing it too(but not guaranteed the second time you launch it/maximise-only happens sometimes).
If you launch Trixx, it won't hang the system if you leave it maximised on the desktop.
Afterburner is fine though, so you could monitor performance with that instead?
AMD should really sort out the ulps problem, some 69 series users had trouble with it, it's looking likely it's following on to the 79 series as well or it could be a zero core issue.
Next to the question mark on the PCI-e info bit on Gpu-z, you click it and it will do a small test and report the proper speed of the PCI-e iirc.