Sounds like a vacuum leak somewhere that's only active once warmed up... egr valve maybe?
Haynes manual says it has and EGR... google will point me places that sell it for your model and it looks just like the one on my '89... Unless someone removed it, there should be an egr valve on the passenger side above the alternator with an elbow shaped pipe going into it and another vacuum widget (EGR vacuum modulator) that connects to it.it doesn't have a egr valve.