Genghis,
Won't make your problem any easier but Rockwell-Collins GPS-4000A units (the common Transport unit) have a current problem that they give random Ground Speeds at rest of up to 14kt - the newer IRUs with automatic initialisation had to be be modified to cope. Apparently will not be fixed until after the WAAS-capable unit comes out in 2007. Wonder if this isn't deliberate wrt to non-navigational uses.