Think I found the solution to your problem...
Based on info from here:
Turn "Digital Captions" to "None" (Settings..Displays...Closed Captioning)
Leave "Analog Captions" alone..
Actually -- I found the opposite to be true -- turning "Analog Captions" to "NONE" and leaving "Digital Captions" at DTVCC1 worked.. My suggestion is to experiment with both settings and see what works best for you -- I do not use captioning on a regular basis, but it seems obvious that you do -- so you would be the better judge to see which combination of settings works best.
Tivo also has an info page on garbled captioning here: »support.tivo.com/app/answers/det···#garbled
They suggest the settings in my crossed out section to see if it fixes the problem.
Hope that helps..