98 Windstar, rpm will vary from very low to normal and then all is fine

I drive a 98 Windstar that drives OK for a few days and then suddenly the rpm will be all over the tach. The motor has stalled on occasion. This
only happens when the vehicle is stationary or rolling very slowly. Then the van can be OK for another few days without incident.
When I am on the highway there is no problem.
Any ideas?
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