Sorry to hear of problems with Sat Nav. I have used both a Navman and a Sony in France and have never had any problems either around Paris or in pretty rural areas.
As for Paris I regularly travel around Paris by any of the three usual routes, (Francilienne , A3/A86 or the Periferique) depending on the traffic reports via the overhead signs and only generally experience problems and long delays due to accidents or during rush hour.
I would also endorse the use of Mappy.com for route planning prior to departure.
Good luck with trip