Usually when this happens it's the memory card.
As the camera is recording frames of video, it goes to the internal memory buffer of the camera. The camera, in turn, writes the data to the memory card. The memory card's ability to "write" data is usually the slowest link in the chain. If the card cannot write data as fast as the camera can capture frames, then the internal memory on the camera will fill up and the camera will be forced to stop recording.
The specs for the T1i require a minimum "Class 6" memory card (the logo look like a C with a number 6 inside) to record video, but frankly a Class 10 card would be better.
I've come to learn that not all memory cards perform nearly as well as their manufacturer's claim. SanDisk and Lexar brands have a fairly solid reputation. Even among SanDisk and Lexar, they make different grades of cards.
Lastly, card do eventually wear out. Think of them like car tires. After one ride you wont notice the wear.... but thousands of miles later it all adds up. The process of "writing" to a card slightly degrades it and after enough writes the card will fail. For a quality card, the number of writes should be very high (in excess of 100,000 writes). So it's possible that a card which was fine once upon a time may start to exhibit problems after a few years.
Tim Campbell
5D III, 5D IV, 60Da
