I assume you have a tuning adapter, hence no signal strength per channel. My guess the problem is either the tuning adapter screwing up or a lost signal from you cable provider. Do you regularly you reboot the TA?
Btw, you can find the signal strength on the tennis channel. Just tune to it and then go to DVR diagnostics and find the channel and its signal info.
Truck roll will likely find nothing unless the channel is acting up when he gets there. Tech: "it's working fine, you need to call TiVo".
