I've got a gsxr front end with magnets mounted on the discs to run the speedo. As shown here -->
http://www.gregoryli.com/search/label/DIY
Since I fitted new discs the speedo reading has been jumping around all over the place. It showed 170mph the other day

. It always seems to be over-reading but never settles, even at constant revs.
I've checked the magnets and they are all the correct way round so that they are attracted to the sensor. And I have checked and re-soldered the wiring connections.
Is there anything else I could check or is the sensor just knackered? It was fine last year, but on it's first run this year (after winter hibernation and new discs) it was wonky.