The time thing tells me it's heat related.
1. 1st check the temperatures of your components ...sounds like something is overheating......CoreTemp, Real Temp will work
2. Does the inside of your PC look like this ?http://www.theregister.co.uk/2009/11/13/ventblockers/http://www.theregister.co.uk/2009/11/06/dirtiest_pc/
3. Are all your fans spinning ?
4. Is your CPU heatsink clogged with dust ? Gran one of these:http://www.officemax.com/catalog/sku.jsp?productId=ARS23935&history=y7dbchkz|prodPage