Erratic speedo on 07 SD

Hi,
My 07 SD speedo seems to have gone nuts. Some of the time it seems to be reading correctly but then other times it seems to go waaaaay off, showing like 100mph when I reckon I'm only doing say 70. And if I hold a constant speed at say 80, the speedo readings go up and down by maybe 10-15mph without the bike changing speed. This happened quite recently and seemingly for no apparent reason. Anyone got any idea how to sort it? Do I need to do some sort of reset?

If it is only the speedo and nothing else going wrong on the dash it is likely the connector behind the dash.
To fix it take of the windscreen, put a rag on the front mudguard and drop the headlight down by removing the 2 screws at the top.
Then take out the self tapping screw that sits under the windscreen and goes in sideways, this allows you to "split' the headlight mask a bit in the middle.
Take the bushes out where the 2 screws fit at the to as they hold the dash in the headlight mask.
Lift the dash assembly out and at the back you see 3 screws, undo and your there.
Remove the connector and clean, just be careful as it has small pins.
Reassemble and your done.

If you have other issues like battery not charging properly and bulbs blowing you could also have a rectifier - regulator problem as it sometimes displays the same symptoms.