Erratic speedo

[TJC46]

Hi guys just been out for a quick blast and noticed driving off from start up the speedo was working intermitently. It kept showing the speed i was travelling at then it would drop to zero and then back up to my speed. If i pressed the accelerator it would read the correct speed, as soon as i lifted off it would drop to zero.
Drove thru town and this was still happening then onto the motorway and opened her up [to the legal limit of course]. As i slowed down to leave the motorway the speedo was erratic again and every time it went from zero to show my actual speed the revs rose and dropped slightly in parallel with the rise and fall of the speedo needle.
Decided to head for home a.s.a.p just in case, and would you believe it speedo started working perfect. Now no matter what i tried including flooring it and lifting off as quickly i couldnt get the speedo to misbehave again.
No doubt if i take the car out tomorrow from cold it will do it all again.
Anybody got any ideas as to what this could be?
thanks Tom.

[Mark.]

Likely the sensor on the diff on it's way out or lost adjustment. Mine did same for ages and then gave up completely.

[EGB]

The interface on the speed sensor on the diff perhaps needs a clean. Its very exposed to road dirt. Mine did this until I managed to push some 320 wet and dry between the rotating faces and to clean and brighten the rotating wheel. This did the trick for me. Need small hands and a good light. Also clean any electrical contacts.