It's not a problem, just a bit inconvient
If you read the manual, you'll find your device somehow has to 'see' the satellites. The more satellites it sees the more exact it nows where on the globe it is/you are. Based on the positions of the different satelites it calculates its position and height, as soon as one or more sats are blocked, permanent (behind the horizon) or temporarly (buildings, trees) the calculations get less exact. When signals get bounced and deflected things can get even more difficult to calculate.
Because of all this satellite detection and calculations it can take long (up to 20 minutes) when you first start your GPS after a long time and/or a big transition, it litarly get's lost (most systems you can help, give it a new initation).
A car, by nature, is a bad surrounding because all it's metal can block but even worse, it can deflect the satellite signals, giving false readings. If you find this happening, an external antenna outside the vehicle can help you, given it has a clear view at the sky.
I wouldn't be very worried about it, if you constantly have one eye on your device, you'll notice this glitches (and get worried), they are mostly short, and most of the time there is only one route/road/track to follow. Look outside, enjoy, use this one eye to glance on your clocks/gauges/warning-lights ánd GPS every now and then.
bart & sophie