....my GO 720 is convinced that I'm driving across fields, sometimes paralleling the road I'm actually on.
That can be down to one of two things... either the lines drawn on the map are not quite in the right place (You often see something similar on Google maps if you look at the satellite view with the roads superimposed)
or.... the GPS positioning on the TomTom is not REALLY as good as you think.
All "automotive" satnavs are actually quite vague about where you are and so they cheat and "snap" you onto the nearest road.
This works well up to a point, but eventually it gives up and decides you must be driving over a field after all.
You'll tend to see this a LOT more if you don't have a route set, because the other sneaky thing they do is snap you especially strongly to the road it EXPECTS you to be on (after all, you'd never disobey it and go a different way, would you?
)
I often see that if it wants me to come off a major road down to a roundabout, but I stay on the flyover on top of it. Because I'm on a parallel road, it assumes I'm on the one it wanted me to take, for quite a long time.
Finally, there have been times in the past when everyone noticed the "parallel road" problem happening more than usual for a week or so.
We never got to the bottom of it, but it seemed like the internal sums were wrong for a while!