I am using the component video outputs of my TiVo HD. Today, immediately after starting playback of a recorded program the screen went blank and the TV put up its "no signal" message. (This message does not come from the TiVo.) The TiVo was downloading a webcast (blue light) at the time and since I've just started using the webcast feature I thought maybe the problem might have been a bug related to that. Nothing I did with the remote control could bring video back. Finally after rebooting the TiVo twice and switching to HDMI video (which I normally don't use because I think it causes lock ups of the TiVo occasionally), I got video back and was able to switch back to the component feed.
Then later tonight it happened again and this time I could get video back by power-cycling the TV. But it would happen again after just a few seconds of video. I had been suspecting the component video output circuitry of the TiVo was intermittent but this is inconsistent with being able to recover video by just turning the TV off/on (since the TiVo has no way of even knowing whether the TV is on or off when using component video). The TV's no-signal message includes the advice to "check cable connections", which I of course had been ignoring.

After all, how could all three video cables become disconnected simultaneously (and the two audio cables also, since the sound was going off too)? But, lacking anything else to try, I started jiggling the RGB component cables going into the TV and vwah-lah! the green cable was apparently making poor connection -- because reseating the green cable (only!) was what cured it.
There is a simple explanation for this, which I finally realized: The H and V synch signals are superimposed with the green video signal and if the synchs are missing, there is no video at all.
So the lesson is: occasionally the stupid user interface messages actually relate to the problem!