Just spotted this.

The problem is DEFINITELY the Vehicle Speed Sensor (VSS) packing up. Better to get it changed sooner rather than later as, at best, the speedo will just drop to zero every now and again but, at worst, the car will totally cut out at whatever speed you are doing (whether that is crawling at 5mph or caning it at 105mph), as I'm sure people can imagine it's better to get it sorted and avoid that kind of risk really!

The sensor is mounted on the back of the gearbox and it can be a relatively easy change BUT if the retaining pin doesn't want to come out or seizes then it's a gearbox off job to fish it all out and change the sensor over!!!

Best of luck with it if you are doing it DIY on a driveway with no instructions!