I spent the last month fighting this issue. It took 8 truck rolls and them sending a supervisor to my home to get it fixed, but the supervisor had an ingenious solution.
My problem is one that I have read about and no one had a good solution that I had read. The setup is a Series 3 HD TiVo with Multi-Stream card and Tuning Adaptor. The card would get associated with the TiVo but it would sit around and eventually time out trying to acquire channel information. TA was showing ready (solid green light) and associated with the TiVo.
Connecting the cable box up without a card or the TA would result in being able to receive basic non-switched channels, but there aren't many of them left in my area. This showed the TiVo was not the problem.
The supervisor found that the TA causes the signal to drop by 9Dbm on the output side of the TA. In my area this was enough that the TiVo couldn't get a strong enough signal to acquire the channels.
The solution still causes a signal drop but not nearly as bad as the TA. You need a 2-way splitter (one cable to two) and a 75 ohm terminator (often called an acorn by the cable installers). The terminator is important or the line impedance will be worse than the TA by itself.
You add the splitter as close to the cable box as you can (a few feet is plenty close) from there, you run one piece directly into the cable-in on the TiVO, and the other piece of cable from the splitter you run directly to the cable-in on the TA. The 75 ohm "acorn" is placed on the cable out of the TA.
Viola! The solution here still reduces signal strength by 3 Dbm thanks to the splitter, however this is 66% LESS signal drop than the 9Dbm signal drop caused by the Cisco STA-1520 TA. After doing this my TiVo suddenly started working!
Reading other issues people are having with the TA's dropping out, this may help those issues as well and would love to hear from people that try this!