Sirius Problem - Help!

I have had Sirius installed in my Prius 2005 since January. It has been working fine, but within the past week or so...when I turn the car on sometimes, it does not list the presets and it does not work. I sometimes have to "reboot" the car in order for it to work. Do you think it's a problem with the actual unit, or the Sirius box that I installed...let me know if anyone has had similar problems...thanks!