Out of synch

I got my Contour box earlier this year after having a previous generation box for several years. Since then, I've noticed that, when I watch a program that I have recorded, after a while, the video and audio get out of synch. I can get them back in synch by exiting the program, turning off the box and then turning it on again and resuming the program, but this is annoying. It's the same television, so I think the fault is in the Contour box. Any ideas how to fix it permanently?