Have you had the closed throttle switch out or adjusted it recently?
Might be the edge of the throttle plate digging into the bore.
Might give the c.i.s a half a turn in and see if that helps matters.
How clean is the throttle body/plate?
A varnishy carbon can build up there and stick things up a bit.
... this usually happens when it's wet outside and/or on the first start of the day, but it can also manifest itself when you're working a trailing throttle on decel and the plate is working against a strong vacuum.