What else do you have running at the same time? Are you sure your system can handle running all the things at once? Are you using a torrent app and downloading at the same time? Open the task amnager and see what else is running that could be eating up a lot of resources that could help provide us with more info to assist you with this.
Just asking what causes it wont do no good. We need some more info before we can get to the bottom of this.