this plagued me for a while... the stepper motor replacement (cheap part - see below) made it better for a while, but it came back. eventually turned out to be two simple rubber vacuum hoses:
pull them off and inspect them for gunk / ageing rubber - if either, replace them.
for the Stepper/IACV, I used this cheap Napa part:
Part #: CRB 21856, about $60.
If neither of those work, there are a LOT of other threads here that go into other possible conditions...