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A good possibility on these vehicles is a failing crank position sensor or failing mass airflow sensor. Try tapping lightly on these components with a screwdriver or other tool while the engine is running, to see if you cam make the engine stumble or stall. While this test won't catch all failing components, it is easy to do and doesnt require any test equipment. If the engine stubbles or otherwise does anything funny consistentle when you tap these components, the sensor you are checking is probably bad.
You can also try doing this tap test to the engine controller (computer). Late 80's/ early 90s GM vehicles had common problems with bad solder joints on teh controller's circuit board. Often, this would cause intermittent stalling/ check engine loghts/ or other random symptoms. If you can make the engine do anything funny by shaking or tapping on teh computer while it is running, then it is probably the source of the problem.