I have a weird problem with the analogue speedometer on my '03 W209 320. When I first start up in the morning once the speed gets above 40 mph the speedo will not go below about 42 and seems to zero at that speed so adds 42 mph to whatever speed I am doing (if that makes sense). If I return to the car later, the speedo will still be sitting at 42 but when I start the engine, if I wait a couple of minutes it will slowly go back to zero. It will then be ok for the rest of the day. The next morning it will have zero'd overnight but the whole thing starts again. This is every day, it isn't random.
When I first got the car, the speedo was sticking quite badly so I removed and dismantled it and found a couple of breaks in the ring gear. I glued these and carefully filed it smooth and it worked really well for a couple of weeks until this started. I had the speedo out again and my repairs have held and the gear rotates smoothly with no tight spots.

Has anybody got any idea what the hell is going on or do I just need to spring for a new speedo?

 

If the gears are good, my guess is that one of the capacitors driving the servo is bad. Note that if you replace the cluster, you need a "virgin" unit and then you have to use SDS to code it to copy the odometer reading from the EIS (ignition switch). A used cluster cannot have it's odometer changed, even with SDS. However, there are shops that can erase the odometer in a used cluster to make it appear as virgin so SDS can re-code it.