Well, my f90bt is doing the same thing. Started after a few days of good "normal" use.
Today it rebooted while listening to an SD card MP3 and using the nav at the same time. Then continuously afterward while driving. I stopped, parked, restarted the car and it all came back on fine. Then when I pulled away and got over about 15mph it rebooted again.
So I came home and checked the board here. Sure enough, I can get repeatedly the same results as described in this post. I checked the wiring and it is fine. Sometimes the unit will not boot up properly at all. Have to shut the ca