Here are some further suggestions:
-
How do you have the grounds connected? For best noise immunity, designate one of the Arduino pins as signal ground. Use it only to connect the ground side of the Hall sensor and the shield of the shielded cable. Use the other ground pin to supply power and for any output devices.
-
I presume you are powering the Arduino by feeding +12v from the motorcycle battery into either Vin or the barrel jack. The problem could possibly be caused by noise on the 12v line. You can check this by temporarily running the Arduino from a 9v battery and seeing if it is any better.
-
Keep all the wiring away from the HT leads.
-
If you post photos of your setup, they may make is easier for me and others to diagnose the problem. It would also be helpful to know what sort of shielded cable you are using and how you have it connected.