Replying to My Canon T3i stops recording.. help +k

I have a Canon Rebel T3i and I'm using a HDSC 8 Gig Class 6 memory card. Sometimes the camera will let me record for as long as I want, but most of the time, it only lets me film for about 3 seconds then automatically stops recording. If I play around with the settings, it will also sometimes let me record as long as I want, and then it will stop recording if I try to film different times again. It doesn't even matter which quality of video or FPS I will be filming in, because it will still do it. Any help would be appreciated. Thanks.