Power Save Mode and 100% CPU Usage problems

Consistently while playing some games like GTA IV, HalfLife2, or Call of Duty4, the game will lock up and go into a "power save mode" (this is what posts on the monitor) for a brief moment and then comes back.  What is more irritating is when the game locks up, the screen goes to black or only shows a few graphic layers, but I can still hear the game running.  I will check the task manager at this time and it is always at 100% usage.  My PC meets all the game requirements specs, so no sure what the problem is.  What can be done about this?...upgrade my CPU?  More RAM?  Any thoughts?  For reference, my PC is a Dell XPS 420, Intel Core 2 CPU 2.13 Ghz, 2 GB Ram, Nvidia 8600 GTS.  thanks for any help.