You should have the globe plus the GPS letters all the time. If GPS is not present, you are not getting a satellite signal.

I would suggest that you drive your car, and when the problem develops, quickly go into service mode and see what is happening.

To access service mode: select SETTINGS, then press and hold LH side of MENU button and wait.

Check your sensor values - wheel sensor, forward/reverse, and gyro
Wheel sensor should increase proportionally to speed. Direction should be forward unless in reverse gear. Gyron should be around 2500mV, and swing up and down as you go around in a circle.

Check your GPS status. If you have a GPS fault, it will show here.
If you do see a GPS fault, then check the GPS antenna connections, check the cable, replace the antenna, and then only when all else has been checked, consider replacing the nav computer (which has the GPS receiver build in)