On a 1.8 zetec what would cause when coming to a stop and putting the clutch in (or sometimes but very rarely when changing gear) the revs drop as normal but dip right down into the bottom of the rev counter and then the engine will stall?

When sitting there ticking over it isnt a problem no fluctuations (spl) in the revs or anything just the issue with pulling up to a stop mostly??


Mart