Sounds like the fan to me. Is the fan making any grinding noises or does it appear to lag at time when running? Replacing the fan is a cheap fix, otherwise you may need to check the voltages on you mobo 20/24 pin and 12v rail to make sure that the PSU is okay. If you have a friend that will allow you to run his/her PSU outside of your case and check the cpu temps that would be a very valuable dx tool.
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