As denbow says most likely a dry cable. Undo bottom fixing and withdraw inner cable clean by pulling through some clean rag the oil lightly with 40 grade oil just enough to give it a film then re-assemble turning the inner so that the square locats into the speedo then fit the bottom end making sure the tongue fits the slot in the speedo drive and re-fit the cross head screw ( Do not overtighten as you can break the plastic also do not over oil the cable as the oil will work its way into the speedo)
Thanks Guys...cable is secure and located correctly,Have removed and lubed with CRC (WD40) and is still erratic.
I will remove again and do as tonupdave says and lube with 40w oil.
Will post results back.
These speedos work by a magnet spinning inside an aluminium disc attached to the needle and sprung against the stop by a coiled hairspring......If the cable turns intermitantly you get a jerky reading... The only other cause is the needle sticking or dry of lubricant..One way to test is to remove speedo (A pain!!) and twist it sharply and the needle should swing freely if it don't thats your prob. More of a pain because it needs to be stripped by a specialist ( The rim is rolled on not like old speedos which screw on)
Hopefully the cable oil will do it...Make sure the cable runs in a nice sweep up to the speedo with no sharp bends..I know it sounds obvious.....Good luck..