6 reasons the dealer installed XM radio in my Prius seems to loose the signal.

1. Turning the car on.
2. Changing directions.
3. Stopping. Or going.
4. Driving on a sunny day.
5. Driving in the rain.
6. Singing along to a song - this seems to taunt the satellite causing it to turn it's back on you.

I finally discovered the actual reason I lose a signal. It works as long as I don't drive east. Finally convinced the dealer to mount the antenna on the roof and it works great now.