Hi Topper
I'm not too sure what your problem could be, but it could be an over heating problem. Can you hear the fans working when the computer is on? Is there a build up of dust and cobwebs that could be obstructing the air flow inside your tower.
Unplug from the power supply and take a look inside. If you find lots of dust, etc, take a small soft paintbrush and very carefully and gently agitate the dust and cobwebs...I repeat..Gently! When you have done this hover up the dust, etc.
Note: Do not touch any component parts with the hover nozzle. May seem a little odd doing this...but it does work...Be gentle, take your time.
Also...if your computer has a low specification it could be that the CPU is having to work harder than it was designed to. What would make this worse is not doing regular housework on your PC.
This would also make the computer run slower. Housework could include running the Disk Defragmenter once every month or six weeks...Cleaning out the temporary internet files on a regular basis...deleting cookies, and so on.
There is a free program call ccleaner that will help you do some of these chores. You can download it from here...Download CCleaner 2.27.1070 - FileHippo.com.
Many people use ccleaner, and I wouldn't be without it. Maybe some of the more experienced members here can tell you more about it.
Another reason for slow running is too many unnecessary programs starting up at boot. Be careful when disabling start up programs. I would suggest taking advice on which one's are needed and which one's are not.
A couple more things that you can do yourself are reseating your memory sticks, and resetting the CMOS. Both of these things have been known to cure the problems you have.
To reset the CMOS remove the small flat battery from the mother board for a couple of minutes and then replace it. The battery will look like a small silver coin and would normally be held in position by a small clip. Note: Either wear a wrist band when working inside your case, or touch the metal casing to get rid of static electricity in your body before touching anything.
These are all the things I would try if I had similar problems...Hope this helps out.