This thread hasn't been here long, but thanks to a link to another thread with a similar question I now have my solution. Thanks to ShadowR34per for his quick response; he may have been wrong, but he was still trying.
The solution is simple: to much heat. In an answer in that other tread, someone suggested a heat problem and supplied a link to download "CPUID HWMonitor" program. This program allowed me to see how hot my processor was getting. Adding a fan (external, as my computer is a laptop) has solved the problem. Unfortunately, I can't remember who made the post that helped me, nor can I find the tread again. Whoever you are, thank you. You may not have answered me directly, or helped the person you did answer, but you helped me.
I have been unable to mark this thread solved. Sorry.