hi ive just had this happen to me....first I went with changing the rotor sensor...it was shot to shit because it wasn't lined up right when wheel was put
back on..(not by me)..in the end bought speedo cable of ebay £45 and hay presto spot on.....but for weeks I thought it might sort it self out
because it was intermittent..would come back on when I turned lights of...then steady for a bit..then go crazy...then
all of a sudden it went and never came back about a week ago...thought it might be electrical but no it was the cable
good as new now
I started with the least expensive rotor sensor....good luck
bought mine from THUNDERROADMOTORCYCLES £45