I have an x910BT that I installed about a week ago in an '03 Nissan Altima.
I have the GPS antenna mounted inside the car, above the dash, as close as possible to the front center of the windshield. When I start the car, the x910BT claims that it can find a GPS signal just fine -- even when started inside a parking garage -- and will let me start navigating a route. However, after about 10 minutes of driving I lose GPS signal (the GPS "radar-style" status screen goes to three yellow lights).
In order to get signal back, I have to either power cycle the unit (by turning off the car