That - um - is a data point you hadn't provided to us before. Indeed, it should and would have pulled all of the requisite info during your drive. Double check WAAS/EGNOS enabled, datum is WGS84. Then if you've still got major issues, please check satellite page for number and strength and report back.
I had a similar experience with my Dakota 10 (10 to 20 meters off, had to navigate to a cache using the map) and I was on the road from Quebec to Ontario. Well, I had the Dakota set to lock on road, and the cache was not on the road. Once I set the gps to not lock on road, it behaved normally again (but it showed me sometimes driving in a lake).
Solved that was my problem, I had it or my wife (hee hee) had it set to lock on the road. Went out this afternoon to find a cache I have found previous and it was within the normal accuracy range. Thanks for the help.