Renault Megane speedometer intermittent

Megane 1999 automatic with ABS, the speedo has suddenly started misbehaving. It's worse when the car is cold, it improves a bit after a few minutes, and only ever flicks high, never low. There seem to be no other problems, warning lights etc.

It also only happens with the car moving, as I jacked up the front and ran it for a while, thumped the instrument panel, wiggled the wiring harnesses to it, and it didn't twitch once. I've already had the panel apart looking for broken joints, and found nothing. Back down and on the road, the problem was immediately there. The speedo movements didn't seem to correlate with potholes, but might possibly have been worse on acceleration. Revving with the wheels up also didn't disturb it, just a nice smooth rise and fall.

Does anyone know where the drive signal comes from? I've seen hints that it's from the ABS rather than a gearbox sensor, and the wiring diagram shows a wire from the ABS box to the panel which doesn't go to a light bulb. Though I don't have a lot of faith in Haynes wiring diagrams. I've also found suggestions about bad engine earth connections, but that wouldn't change with the wheels up.

Up to about March 99, definitely a speed sensor, after that probably a feed from the ABS system. I've been using the wrong manual for six years without noticing, so not much else is different. The instrument panels look the same in the pictures but aren't, the later ones having LEDs rather than bulbs, and different connectors.

It was a bad engine earth connection i.e. a broken wire. Strange alien pulses were appearing on the speed sensor line in addition to the proper ones, particularly on acceleration. I still don't know why the speedo was stable with the car stationary, because the extra pulses were there. Possibly they were too low to cause trouble, and increased in level a bit with the engine actually having to produce some power.

Specifically, near the top of the block at the rear on the offside, there is bolted a large braid which leads to the chassis, and under this is another bolt holding a crimp tag with two black/brown wires coming from the adjacent harness. One of the wires was broken, although this could not be seen in place, and I only found it having taken off the tag for wire-brushing.