can be cause by a few things. most not good, like malware, or your antivirus updating.
scan with aq good av. check your task scheduler to make sure your not getting some kind of update or interruption. open task manager and see if you have any active processes that may cause a screen swap.
get a process monitor which shows what processes are being run. and run it while you play the game... you should then see a constant stream of the game name interrupted when the game minimizes.http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx