I believe a vacuum leak could cause this and the suspect areas are the boot under the fuel distributor, the IAC hoses and the 90° elbow from the intake to the valve cover. As good measure replace the hose from the top of the air cleaner to the valve cover and the oil dipstick O-rings unless they are excellent. None are expensive but can be tough to install.

IIRC Duke has dug deeply into these systems.

Good luck!!!

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