You have mentioned that the cable company has implemented new products. These new products could be encroaching on the frequency range that you are using for your modulator. Even if their explicit ranges are below what you are using for your modulator, the cable companies are notorious for noise.
My suggestion would be as follows - you say that you are using channels 104, 106 and 108. The frequency for these channels is above 649Mhz in the spectrum - this SHOULD be above cable, but ... You can buy a filter from Channel Vision
. Look at the 3102-105 filter. This is a low pass filter that will allow all frequencies below channel 105 to pass, but provide a clean frequency range above that, which can be used by your modulator. Put the low pass filter on the line from the cable input but before the VDU.
To test, once inserted, channel 104 should disappear from the sets around your house, but if effective, channels 106 and 108 should still be present and clear.
Now, with respect to the TiVo and clarity of the text, I don't doubt that your picture was clean on the RCA connection, while dirtier over the coax. There isn't too much I can suggest here. The modulators will NOT produce as clean a picture (at any distance, regardless of noise) vs. a direct connection. However, I have found that the quality of picture that I get with Channel Plus products (particularly the 54xx or 56xx series) is excellent. Further, they have built-in amplifiers that produce a bright, strong signals.