i found the built in gps receiver isnt' all that strong. i find that when i'm inside, it'll get the signal strength to get a lock on my location.
make sure you're external gps setting is com 4, hardware none and baud is 4800 and check managed gps automatically.
i also use gpstest to see if i'm having problems with tomtom or my gps. can be downloaded here
http://www.smartphonefreeware.org/do.../gps-test-1.04