PowerCool are not that great TBH. Is it possible for you to borrow a better PSU from somewhere to see if your issues persist??
Also,what motherboard do you have?? How much RAM?? At least 3GB to 4GB is a decent amount to have.
Have you tried monitoring the temperatures of your CPU and GPU??
Install this application:
http://www.cpuid.com/softwares/hwmonitor.html
See what temperatures you are getting.
IMHO,there are a few possible problems which
might be happening:
1.)Your PSU is not good enough for the job
2.)Your motherboard VRMs are overheating due to lack of decent cooling
3.)System is overheating due to insufficient cooling