Your problem sounds like it could be related to the vehicle speed sensor. This sensor attached to the gearbox informs the speedometer and ECU of the vehicle's speed.
If this were to fail, you would expect the speedometer to read zero and perhaps the ECU to be confused, causing the problem.
To repair costs about £50 this problem wont effect your car, but it has been known to cut out the engine at low speeds.
Its more of an annoyance than anything. Best to get it fixed though. ds
Dave Sanins, March 2006