Channel creep

Starting a thread is presumptuous, but I can't find anything like this:

My digital cable box (Motorola DCT) receives eleven premium channels -- mostly the offerings of Starz and Encore -- with numbers between 722 and 763, which I've come to think of as the "no-fly zone." My Series 2 TiVo (a Toshiba SD-H400), if programmed to record any of the channels in the NFZ, will tune the cable box (via serial cable) to the proper channel and begin recording, whereupon the cable box will begin stepping through the available channels -- upward if it started on a lower number, downward if it started higher -- not stopping until it reaches either 722 (which I receive) or 763 (which I don't). At that point the TiVo's "display" button will still indicate it's tuned to the desired channel -- but the recording will show the brief stops at all the channels, then lock in on 722 or 763. If I tune the cable box manually to a channel in the NFZ, I can then record it manually. But setting up a recording in advance produces this "channel creep" even if I preset the cable box to the channel I want. And tuning to channels in the NFZ using the TiVo remote (without attempting to record) results in creepiness as well.

I have done a full reset of the TiVo settings, wiping the drive (insofar as I can). I happened to have an identical TiVo in storage, so I've installed it, then done the full reset routine again. I have swapped out the cable box. I have examined and tinkered with the settings of the cable box, including switching "Auto-tune" off and on and looking for any lurking parental controls. favorites lists, etc. For what it's worth, an HD DVR elsewhere in the house displays none of this bizarre behavior.

Anybody?