Older computers can frequently have heat related issues from hard use over time. Make sure you've used compressed air to blow out all the dust and junk from inside the computer, around the processor, fans, etc. Look at the motherboard to see if any capacitors are buldging, swollen, or leaking.

Next, make sure all background start up programs and services are limited to only those strictly necessary. When running games, temporarily disable your antivirus program. Depending on your operating system, run Scandisk or Chkdsk/Error Checking and then use Defrag to optimized the hard drive..

Hope this helps.