My VSS isnt hooked up right now and the gps is still fairly smooth. It also picks up my speed very accurately. Try going into system check when your driving at slow speeds and see how fast it says your going. If its not telling you exactly or very close id have the shop swap it out for a new unit. also where is your antenna mounted
My antenna is on the roof. We gutted an XM antenna and put the GPS inside. The car is in the sound shop now, they are looking at the speed pulse and are going to get into the menu.