Canon EOS 550D video
The reasons for an automatic stop are either reaching the 30 minute limit (thank you European Union video camera tax that ends up impacting what's available to most of the rest of the world), hitting the 4gb file limit (file system limit for the OS on the camera), the sensor is overheating (high usage + temperature) or the buffer is being used up (memory card is too slow).

Based on your description of the problem, the first two do not sound like the issue, however you could either be hitting an overheat or a buffering issue.  A class 4 card is a bit slow for recording HD video, but I'd still expect more than a few seconds, particularly for a SanDisk card which should actually be giving you speeds for a class 4 card.  (Some less reputable manufacturers routinely sell cards marked faster than they actually test in the real world.)

I'd first try getting a faster card and if that doesn't work, I'd make sure you are using the camera in a cool area and see if that gives a longer run time.  It's also possible there could be something wrong with the camera that is complicating things, but a slow memory card is most likely the problem.