The PSU seems good enouph for this gpu. It shouldn't be the problem since other games run fine. Stress the gpu with Heaven 4 benchmark for at least 2 consecutive runs to know for sure (yeah, i know, i am a geek), and monitor temps and rail voltages (gpu-z, hwinfo, afterburner).
The anti-aliasing on AC Unity can be stressful, so try without it.
In any case, monitor temperatures, voltages and clocks, and report back anything unusual.
Are you sure that the integrated intel HD 4000 gpu does not kick in ?
What catalyst drivers have you tried so far ?
check in the BIOS to make sure the Intel integrated graphics are disabled