Problem reported by user in this thread which I also experienced:
Using FF in a recorded episode, especially at fast speeds, can't keep up with what you see on screen.
Workaround (pretty cool):
from the guide, page 35:
If you want to jump to a specific point in the recording, Absolute Jump allows you to do just that. Use the number keys to enter the point you want to jump to, and then press PLAY to move to that location. For example, pressing 4, 5, PLAY will move you to 45:00 in the program!
This is OK if you know where you want to skip to. You can also use xx minutes skip forward. But if you want to be able to see what is happening, you would have to skip forward in small segments. This defeats the purpose of having a very fast forward function. An example of this is when the men's final of the US tennis open was rained out yesterday, they showed football for about an hour, then switched to tennis commentators and then to a semi final replay.