Well, what you have done would seem to point at there being a hardware problem. The likely cause of slowdowns is temperature. The first thing that I recommend you do is download and run the Intel Processor Diagnostic Tool (here's a link for doing so: Download Intel® Processor Diagnostic Tool). It includes a stress test that will determine whether there is an obvious issue with temperatures. If this passes, then you want to watch temperatures while running a more significant stress test. The best stress test software I know of is Prime95. You can download it here: GIMPS - Free Prime95 software download. Here's an article that will also help: Torture test your CPU with Prime95. While you run this tool, you should also run something that will provide you with temperature readings. Open Hardware Monitor is useable, but there are better ones like AIDA64 (download here: AIDA64 - The Ultimate System Information, Diagnostics and Benchmark Tool), which has its own stress test built in (this is a pay tool but you can trial run it for free).
Hope this helps,