|"my computer shuts down after 2 hours of operation"|
Could be caused by overheating. See if it will restart after about 15-20 min.
"the fan of the power supplly is not working...i just replaced the power supply 1 month ago..."
Is it a high quality brand name unit with proper amperage distribution costing at least $50-75? Or a no-name cheapie costing closer to $20?