is there times that the stock nav just acts weird and displays the car across town like a different location? the reason why im asking cause my brother '00 MB ML bugs out like that times and we cant figure out why. btw he has a cd based system. something wrong with the gps antenna or the disc?

 

Never noticed anything extreme like that.
However, if the nav doesnt have a GPS signal, and I get off at a freeway exit, sometimes it will think I have kept going and show my position wrong. But that can be expected..