Though it may not be the case here, a leaking pcv valve hose (the elbow connector on top of it) will cause this symptom. It would be wise to check your vacuum hoses as well, since at this advanced age they tend to be rotted and leaky without regard to how many miles are actually on the vehicle.
I had replaced the elbow with a different type of hose which fit and within 6 months it had become swollen and popped off (due to oil contact) causing the same idle problem you mentioned. I would definitely suggest a trip to the dealer for the part if that's the issue you're facing.
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