01 929 Speedometer works when it wants to, HELP!

Hey guys, I just got my 2001 929 yesterday... i got it with a bare back tire and the speedometer only works when it wants too... I got the back tires replaced last night, now im working with this speedometer issue. I have replaced the speedometer sensor that goes in the top of the engine, no luck. It will work for like the first 10 seconds of the ride, then it goes down to 0. then while your riding it might jump back up to the true speed, but it only stays for a few seconds. Any help would be appreciated... I need to get this fixed asap. Thanks guys, I look forward to being part of this forum.