I don't know WA. I can't trace the problem. It started shortly after the latest Terragen update. Since there has not been other people bringing this up, I am guessing it's a hardware problem.
I was going to re-build this system to make it more current anyway. New mobo and processor, with a power supply able to handle more hardware options.
Try running some benchmark software. Also, something which will trace and stress test the CPU/s. It may give you a clue.