The speed chip went on my Voyager and the engine would just cut out intermittently. After a few minutes it would restart....until the next time. My local dealer identified the fault as soon as I took the car in. The likelihood of a speed chip failing is very small (like the chance the chip in your PC will fail), but I guess some do.

The man from the RAC thought it was the chip in the key fob not being recognised, which could be another reason.