Same thing happened to mine.I saw the Watchdog programme and thought it could be the clocks .Investigated further and turns out it was the Speed sensor that sits in the top of the gearbox.£18 but a bugger to change.
It wasn,t just the clocks that were effected,it was other things that were controlled by the speed sensor like: automatic door lock when pulling away and also the stereo getting louder the faster you went.