Faulty NPS
 

would a fault nps cause the car to occasionally stall and lack power?

Yes it can cause problems like that as it's sending an incorrect signal to the ecu.

Thanks,

Anyway of checking without replacing?
Seems to of highlighted itself since i've changed the bov to a standard re-circ

The switch has most probably worn down as it's plastic so you can only see it when you remove it.

It's a ramp job,drop the gearbox slightly to get your spanner onto it (it's on the top of the gearbox) you may need to have small hands though.

I use a OBD lead and Romraider to test the NPS, when you log you can choose the switch that starts it and stops it, If you choose the NPS you can see it change from 0 - 1 if it is working when you change from N to a gear,
A short drive should show it doing the same.

hth