Acer :: 6920G Performance Halving Randomly

my several-month old 6920G (running on vista 32bit, no hardware upgrades) has had a problem for the entire time i've owned it. Its performance seems to halve temporarily whenever i run a demanding program on it(Battlefield 2 at medium-high video settings), and it switches back and forth between normal and half its performance, staying at half for ~10-20minutes and full for 5-10, on average.

The laptop already has most of the annoying acer programs disabled as well as DWM, anything usually safe to remove.

Is this problem caused by stressing the laptop out too much? I played the same game with slightly lower settings, on my (equally computer-inexperienced) friend's sony vaio laptop(cant remember the model) with the same amount of RAM and 1.8GHz processor, and the performance was constant! (and at times better 3: )

Also, ive noticed that my advanced power settings on High Performance Plan changed on their own to default the 2nd time, and the lowest setting of the processor power management the first time.. although it hasnt happened since the first month, and the performance issue is still biting me in the behind..