Set MacBook Pro on energy saving mode
The temperature drop when unplugging the power can be attributed to Intel's SpeedStep technology, which will conserve energy while working on battery by dynamically decreasing/increasing the clock speed.

There is no way to control this from Mac OS X itself, but for your processor you could use the 3rd party software CoolBook. I have no experience with this software though, and I'd be cautious, there really is no good reason to go fiddling with these settings manuals.