One other thing that can affect it, if you have idle set with a known good tach, is an aftermarket cooling fan.
If it is not wired correctly, i.e. a relay that for sure drops it out of all the elctrics when the key is "off", the motor becomes a generator, and as it spins down will back-feed power into the grid, and keep it running, especially if it's on the "ign" side of the switch.