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I would suspect a vacuum leak problem. Those engines are notorious for having a problem with rubber vacuum fittings on the far rear of the engine. With the engine idling. put the hands back there a feel around for the lines and fittings. You will hear the difference of you touch the hole. It's usually right in the corner of the rubber.
Also take a hard look at you ignition components like the plug wires. Ho long have they been on there. If over 50K, replace them.