Hi rfb,
The same thing happens with my gps 3760T, I returned back to the original condition and got this error too, as there was no way to recover this additional condition, back again for 34xx, I noticed that this happens every time I turn the gps, long before stabilize the location in which you find yourself, after a few minutes back to correctly measure, still has an error greater than before the change is more acceptable, the error is around 5 to 6 meters, still have not found anything on how to improve it good luck and post any news for us.