I think it has more to do with the unit than the antenna. I also sometimes notice that the GPS takes longer than expected (indeed sometimes a few kilometers but never 10) and sometimes the position is just of by 30 meters or so. Note that I am using the antenna which was already installed in the car. It was originally supplied with the WinCE unit that I had before.
So replacing the antenna might not be the miracle cure you are looking for.
What does help is resetting the GPS app on the unit. You can find the GPS app in the main menu and there is a reset button. This will force the unit to re-acquire the satellites after which the unit is behaving normally for a couple or weeks/months.
It's just a small blemish on the unit's track record. All in all I am still very happy with it
