Hi,
May be that the signal coming from the sensor is not clean? (I'm having a similar problem with an encoder: I guess it has to do with 'noise' -see in google 'electrical noise' - images-).
If the 'edges' of the signal are not clean the arduino 'sees' several 'RISING' (or 'FALLING') interrupts every turn.
(By the way: what is a POV clock?)
Regards