iPod iPod Video Playback Degradation?

I've started watching videos off my iPod on my tv using a camcorder cable. The first hour or so everything looks great. During the last half of a movie though, the picture will break up slightly even where there isn't movement in the image, then get better, then break up again. It's not terrible, but it's annoying and distracting, because it happens at a steady pace, maybe once a second. What's going on, and is there anything I can do?