i wouldn't count the dealer out without at least asking them - you're pretty close on warranty and if they can't help there the service desk may recognize the symptom. search here on "speedo" for discussions on problems but i think they've mostly been erratic readings cured by diddling or lubing the cable. Have you checked at least to see that everything is fastened tight and not binding anywhere? Is there anyone around you could swap cables or speedo heads with?
I must say, when I first read your post about how the speedo went smoothly to 120mph in first gear I was pretty impressed - it must feel great!
added:
this thread has some suggestions