When it's unplugged, the system isn't running flat-out in max performance mode -- it's at a lower CPU and GPU speed, possibly also with only one core active.
When you plug it in, the system attempts to run at full performance as demanded by the application, generating more heat than the system can handle -- and causing it to throttle back.
Change your power settings: turn off max performance, and disable the multi-core operation [most games don't use multicore processing anyway]. Throttle the CPU speed back -- yes, that sounds counter-intuitive, but the simple fact is that you cannot run anything but the highest-end gaming notebooks [the massive 17" systems with correspondingly massive heatsinks] at full power for more than brief periods of time without having the system throttle back.