Your throttle plate is probably sticking to the throttle body when closed --
you can spray a little throttle body cleaner onto a rag and clean the edge
of the plate and where it seats on the throttle body, once you get the crud
out of there it shouldn't stick any more.  Should also smooth out your
engine a bit when you are using minimal throttle.

It wouldn't hurt to take a look at the throttle cable and linkage too, just
in case something is binding as well.

Hope that helps!