I'm willing to put my money on a compatibility bug somewhere. There's a feature where when you use a sniper scope, the game slows down to half speed. It seems your game is misinterpreting something, thus causing the slow mo.
Try using this Coalesced.ini. Go to your Mass Effect 2/BioGame/Config/PC/Cooked folder and place the .ini in the .zip in the folder.
http://www.megaupload.com/?d=YA0A26C5
Please be sure to make a copy of your original Coalesced.ini file first.
To use this, press + on the keypad for double speed, - on the keypad for half speed, and * on the keypad for regular speed.