The two issues very likely are connected. The serial communication bus connects all the modules, and the engine management system manages your ignition timing and idle speed control differently if you're moving vs rolling. Often the engine will stall when coming to a stop when the speedo has failed. First define what you mean by "not working" on the speedo - not registering speed? Odometer not displaying or not advancing properly while riding? Backlight not lighting up? Needle whipping about violently? Cure the speedometer issue and there's a good chance it will run better.