Thanks for spotting that (I know very little about the kernel).

The capture card I'm using is a Hauppauge 1187 WinTV-HVR-1250, and I've used it for over a year with Mythbuntu 11.04 and 11.10 on similar hardware w/o any problems.

Both this new box and the previous one are amd 64 bit cpus; the only difference is the video card: nvidia for the old box, ati for this one.

I'll try either downgrading to 11.10 and see if that makes any difference, or, if not, switching to the bleeding-edge latest kernel.