Hello here is the homepage:

http://www.asus.com/Motherboard/M3NHDHDMI/#support_Download

Your power supply is probably failing, or to small for the system.

Are all drivers and the bios upgraded ???

If you are useing the onboard graphic, try to add a better graphic card on the mainboard for better performance.

I suggest you to buy a 500 watts power supply, or bigger depending on the systems specs.

Good luck