|First, the best way to overclock or even underclock is through BIOS settings.|
Why don't you address the real problem here which is the overheating? If the machine is not new, you should blow out all vents and heat sinks with a can of compressed 'air' sold at computer and office supply stores for that purpose. Are all fans running properly? Are all vents clear? Is the CPU heat sink properly sized, properly secured, and have the correct amount of thermal paste applied? What are the actual temperatures (idle, 15 minutes working, 10 minutes gaming)? Use HWMonitor and report results. List the make and model (mfg'd system) or all significant components (custom rig) as well as the number, size, and orientation of fans for better advice.
You have to be a little bit crazy to keep you from going insane.