Try this. On the back of the tach is a setting where you tell it how many pulses your stator puts out. In your case you have a 12 pole stator producing 6 pulses. Using a screwdriver move this setting back and forth several times stopping exactly on 6P (close don't count). Then try it again. What happens over time is the contacts inside get cruddy or whatever and a lot of times you can clear it up my moving that setting back and forth. They are very small contacts and that is why the setting must be exactly on.