another thingy , get some soft that can monitor your PSU voltages under load , if you see big spikes on 3.3 and 5 and 12 V then verify if the thermal compoud dont lay over the contact bridges on CPU < sometime it hapening and hapened to me
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and just for sure , please borow somewhere a 13 micron XP CPU , and if is not change then only MB , or Video Card defect can be