Have you checked the temperatures of your rig?http://www.cpuid.com/softwares/hwmonitor.html
Run HWmonitor while you're playing 2 clients and see what happens, then run a third and check if the GPU/CPU are close to their thresholds.
Does it only do that with DAOC? Have you tried other games?
What is your vcore for the processor? And have you overclocked it at all?