Well it depends what windscreen you've had fitted so they may not be telling porkies. Some cars have windscreens which prevent most of the radar signal passing through to be received properly inside the car. This reduces the effectiveness and therefore the warning range of the radar detectors and GPS.
"Athermic" windscreens cause problems for all radar detectors and also GPS systems due to the high metallic content of the screen. Athermic screens are sometimes called heat reflective, solar or heat insulating windscreens.
Athermic windscreens are easy to spot as, from about 50 yards away, they appear to have a "Petrol Blue" haze.
Above was taken from some technical site though I was already aware of it.. Renault screens use this type of windscreen. Check out the next one that approaches you and you may see the blue tint.
Personally, I've had no problems with using other GPS kit with heated screens, well with Ford anyway.
Are you losing the GPS signal? Does the GPS icon grey out? I would not have thought losing the signal would cause the program to lock up as it happens all the time with normal use (under bridges, flyovers etc).