968 Speedo not working correctly

[asj968]

Hi I have a 968 with a partly functioning speedo. The vehicle is left hand drive. What driving along at 50 60 70 etc the speedo only registers 20 - 30 kph.
It's started showing the correct speed for a while but has now got lazy again. I have changed the abs sensor on the front left wheel but it's no better.

Any ideas?

[Waylander]

its the frequency converter in the fuse box that's duff