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!