I would start with the really simple/cheap stuff first. Have you inspected the vacuum lines? I just replaced all mine for about $8.00 and a half an hour of my time. Smoothed it right out, should have done it a year ago when I got the car.

Does your car have the Diagnostic Module with the LED and push button up by the battery?

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K