it will probably be the sdcard, reason being when the Nav5 memory is full it automatically uses the sdcard for the overflow, and a dodgy card or bit of data not written correctly will give a reboot, also give it a good overnight charge on the mains with the USB cable.
If it only happens when it is the mount, it may not be fully locked in place. When it is in, look between the top of the nav and the mount. You should not be able to see any red on the latch.
Also worth checking the nav mount pins are free to move in and out (they are spring loaded) and clean.