Do you get a check engine light? My sisters VX is having a similar issue and is complaining about the speed sensor. Using a elm-327 and my trusty Android you can see the speed bouncing between 20-100 mph every second even when at a standstill. "Rebooting" the VX fixes it for a bit. The sensor is petty expensive (over $250 for a Napa special) and you have to disconnect the auto shifter from the tranny to get to it.