It could be either the TiVo or the cable delivery chain. Even live video is actually a recording (which is kept for a half hour by default) so it is very unlikely this problem only exists when you view recordings -- it would also be there if you had been watching live when the show was recorded.
What cable company? Was the problem always there, or did it start after a while?
The sticky thread on the HD
is must reading for any HD owner.
Sorry it's too late to help you but the consensus on this forum is that the My Expander drive is relatively trouble prone and it is much better to use a large internal drive (such as the TiVo HD XL has). Sounds like your stuff should be under warranty. If not, and the problem is the expander drive, it is relatively easy and inexpensive (less than $100) to replace the internal drive with 1 TB. See the Drive Expansion sticky thread
for all the details and instructions.
You can test if the expander drive is the problem by "divorcing" it and seeing if the problems are still there. Unfortunately you lose any recordings you have saved when you do this.
Edit: Here is a link to TiVo's support page on troubleshooting digital cable signals.
. In particular on the DVR
Diagnostics Page check the Signal Strength and the RS errors, Uncorrected and Corrected for the tuner tuned to a problem channel.
Signal strength below 75 might indicate a problem. RS Uncorrected Errors growing at the rate of 1000's per second are also a problem. Normallly these signal problems would have to be corrected by your cable co., although weak signal could be caused by splitters or attenuators if you have any installed - or even a bad cable or connector at your end.