The only likely reason why a GPS would regularly fail in a particular spot is that the unit is affected by high power radio signals.
This is why the GPS aerial/antenna should always be roof mounted.
The handhelds with integral aerials (e.g. most Garmins) are probably hanging on by a thread most of the time but they do it pretty well so most people are not aware.
If I was using a handheld regularly I would connect it to a roof mounted GPS aerial. Every plane in which a GPS is being used should have one of those fitted, IMHO.