what video card does it have? or is it integrated? If its ATI, then you should download ATI Catalyst Control Center from the ati website, this will monitor its temp, then you can post it to us here, and you should also download Speedfan from download.com, this monitors your motherboard, hard drives, and cpu temp, and then post them here.
If the heat isnt the problem, then it could be something with the OS, when did you last reformat? you should run anti-virus/anti-spyware programs and run disk defragment.