A vacuum leak is a strong possibility in a car that old. There are vacuum lines under the intake manifold, and as the car heats up they soften. Changing them all is a smart way to start. Next try cleaning your mass airflow sensor with the correct cleaner. Its delicate, so don't touch it! Just use the spray. If that doesn't help I'd try changing the throttle position sensor, an easy fix (if that's the problem) for around $75 at Rock Auto.