Have you tried cleaning dust and fluff out of all the fans and that includes cpu fan and graphics card fan where fitted? Disconnect from power and use a soft long hair brush on fans. If you then get a can of compressed air, liated here as Ambersil Air Duster,click here and blow the dirt away. Use air spray as lightly as possible as you can get some condensation from it. Leave for half an hour to ensure that it is perfectly dry and then restart computer.