Tll us what software version is fitted to the nav - it's displayed in the top right corner of the "Set" screen.

The current version is 3-1/63.
Anything older (3-1/50, 3-1/40 etc) should be updated, as the software updates fix software bugs.

Once you have confirmed you have the latest software, please go into the Service Menu and report to us your GPS status.
To access Service Menu, go into the Set screen, then press and hold MENU until the Service Menu appears.

Report the GPS S/N readings, how many satellites are being receiver, your PDOP value and the GPS status.

You should also check the sensors:
Direction of travel should be Backwards in reverse, forwards in all other gears
Wheel speed pulses should be 0 when stopping, and with speed
Gyro should be 2500 +/- 100 when still, and go down when turning left, up when turning right.

And remember: when entering an address, you must select "Start guidance" to commence the route calculation.