Motorola DVR fast forward is erratic. I have noticed the problem seems to only be on HD Cable-Only channels like MSNBC or CNBC. HD network programs on (CBS, ABC, NBC, ...) work OK. Standard definition Cable-Only channels work ok. So I record Cable-Only channels in standard definition. Must be something different in timing information or the processor chokes on expanded data stream.
The non-broadcast channels are now using MPEG4 compression, which allows Comcast to cram 10 HD channels onto one frequency, versus the 4 HD with the older MPEG2. The Motorola DVR's seem to have issues with keeping track of the embedded timecode with MPEG4 data. Local channels are passed through as-is and are still MPEG2, as that is the only digital format currently used for broadcast.