Hi Jim,
I might or might not be on the right track here, but depending on where your living, you could be experiencing a problem with the powersupply into your premises.
Are you currently utilising a UPS or power stabiliser of some sort?
I have seen problems with my own computers when we have experienced brown-outs (or when the power drops by some 15-50Volts in one go, then comes back, this might only be a drop of 5v in your case, but enough to slow down the CPU and cause the computer to have problems..
Alternatively, I also had a problem where due to lack of HDD space I installed games, then removed them when I was bored, reinstalled them later to play again... thus cluttering the system with unused DLL's and Registry entries. You could try a system cleanup to sort this problem out.
I might also suggest a very good program that I have here (I use it often when I go to do checks on client computers, it does computer analysis in a very thorough way and will give you full reports of the analysis.
Sisoft Sandra (I have ver 2001 Professional) but they may have something more up-to-date on their website: (http://www.3bsoftware.com/sandra) also (http://www.sisoftware.demon.co.uk/sandra)
You can apparently download a 'shareware' (or standard) version of this software, or purchase the professional, my version is a little old (2 years) but it works fine, it's the professional version. Highly recommended for finding problems with the hardware or software, and will give you a good report of it in the end, can be very useful when you want to purchase second hand machines as well.
Let me know how it goes, and if you need any more help.
Simon