Navigation System behaves erratic with no GPS reception
 

I have used my nav system for 20k miles with no problem. Suddenly it starts to behave erratic when no or
bad GPS coverage is present. I.e., in a tunnel the little arrow proceeded 90 degrees to the course as soon as the
GPS reception was lost until it crossed some other street on the map, then it followed this street almost
opposite to the actual driving direction. I know the nav system should continue using wheel sensors and the
accelerometer for dead reconning. Now I'm concerned about failing wheel sensors, since they feed more than the
Nav (primarily DSC/ABS/...). But maybe it is just the accelerometer.

I searched, but it appears nobody had reported this kind of failure before. (Truemagellen, are you listening?)

I need to bring the car to the dealer next week anyway, but I would like to give more information to them than
just the failure description.

Thanks for any ideas.

Q-chan