are you over-clocking anything GPU, CPU, memory...if yes try default values
may be dust in your graphics card...you can get a can of compressed gas duster and give the inside of your computer and graphics card a good cleaning
has this started recently? if yes what has changed recently any new programs or drivers loaded recently may be causing the problem
try older Catalyst versions and see if they make any difference because ATI has had a lot of trouble with BF:BC2 in particular with recent driver releases and has had to issue hot fixes/application profiles to address the issues...
Rage3D is a good resource for ATI/driver related info
check what programs you have running in the background, in the past whenever I had a game minimize by itself it was usually caused by some program running causing the game to be minimized and switched to the background