Handheld GPS units can be vulnerable to saturation from other radio transmitters. I used to occasionally get that in the vicinity of the old Rugby/Daventry masts.
Using an installed ariel that only has a view upwards, completely cured the problem as it was shielded from most ground based sources.
I think the RF rejection is better on some of the newer units.
Another possibility is the illegal use of low power GPS jammers by commercial drivers so that they can't be remotely tracked by their employers. They don't have much range, but may be a possibility.