Since this is a desktop system looking from the front of the system take the left side panel off. Check the fan on the motherboard that is on the cpu. Is it dirty or running slow?
Turn the system off and use a can of compressed air to blow out all the dust. Pay close attention to all vents and fans.
Once done check the fans again after turning the system on again. Make sure fan on cpu and the fan in the PSu are turning correctly. Does this correct the issue?