Peugeot 406 coupe speedo doubling speed

We have a gourgeous deep red V6 SE coupe and love it. Only hassle is that it doubles the speed after 45mph, jumping to 90mph. Similarly it flips back to normal on the way down gears. Worst bit is it doubles recorded mileage!

Dealers have been no help at all, changing anything they can think of. So far we've replaced the speedo sensor, wiring from the sensor to the engine ECU, re-soldered all the contacts in the speedometer, all to no effect.

Any suggestions?

thanks
Puggo

Did they look at the the ECU itself?, it might need to be recalibrated or replaced