I have installed .38 and the blackbird drivers...
When I watch TV (I haven't recorded anything yet), there are regular pauses in the vision, perhaps as much as twice a second maybe less, but the interval appears to be the same. The effect is more noticable on fast action and panning shots.
I am using the built in video card, I don't think it's accelerated, so could that be the cause of this "feature" or is it because the drivers are new ??
Check the CPU utilisation on the machine whilst you are watching TV.
The easiest way to do this is to go to another screen (ctrl-alt-F2), login and run top. I assume that you are using MythTV, look for the MythFrontend process and check its CPU usage.
If it is close to 100% then this could be the cause of your problem and could probably be rectified by offloading some of the video decoding to a dedicated (accelerated) video card.
I found this and put in a GeForce FX 5200 and installed the nvidia drivers and it rectified what sounds like a similar problem to what you are getting.
Hope that helps.