Major computer slow-down during games

Hello all. I've had this Studio XPS since it was new in 2009, and over the last year or so, it massively slows from normal 50-80 FPS gameplay to 0-2 FPS. This happens randomly, for variable amounts of time between a minute to up to 15 minutes. Computer activity remains the same as normal, as I can tab between programs and use them at regular speed, but with the gameplay it will remain slow between tabbing in and out, or just letting it fix itself. Some examples of games this happens with are: League of Legends, World of Warcraft, Diablo 3, DC Universe, and Star Wars Old Republic.

I have updated drivers, run several diagnostics using the Dell Diagnostic Tool, both overall and focused gameplay modes. No problems have been found. I rarely run multiple programs at the same time as gaming, in an effort to reduce system strain, but the only consistent program I have on in the background is a virus scan/firewall program. After the problem lasted a month or so I even switched to a different brand of firewall/virus scan, and no improvement was noted.

Obviously, a fairly frustrating issue. Has anyone else experienced this, or have suggestions on how to fix it? A few computer tech friends suggested it may either be video card related, processor related, or motherboard related.