It's weird that the CPU usage is going up. You probably have something running in the background. Spyware/trojans, or just programs that start when Windows loads (startup processes) that don't need to be running but set themselves to. (sorry if you know this, I don't want to make you feel dumb but I don't want to assume you know everything, plus it's late at night and I'm bored so I just type lol)
With my mobile AMD processor, when I unplug the laptop its speed goes from 2.2GHz down to 800MHz. This is called Thermal Throttling, I believe. I think Cool 'n Quiet is responsible for this or something.
Phenom II X4 925c2 @ 2.80GHz (95W)
OCZ Platinum PC3-1333 2x2GB 7-7-7-21
Raidmax RX-500AFCheck it out.