I would check the resistance of the spark plug wires (each wire should have less than 1,000 ohms resistance for every inch of wire length, ie; a 15 inch wire should have less than 15,000 ohms resistance); or just replace the wire set. I would also check for vacuum leaks, cracks, or loose connections in the signal hoses to the MAP sensor (shown in photo here: (http://www.rockauto.com/catalog/moreinf
This motor had a special design spark plug made for it by Champion; who supplied the original plugs. This plug has a substantially longer insulator than most plugs; which improves idle stability. If your plugs have more than 30,000 miles on them; or have been replaced with a different brand; I would install a new set of Champion # 436 (RC12LC4). Champion also makes a premium performance Iridium plug for this motor under # 9203; which would have an even broader heat tolerance.
The air filter element should be clean enough to pass light easily. A restricted air filter can adversely affect the idle quality. And the throttle body should be thoroughly cleaned; particularly the front and back surfaces of the butterfly and the surface of the bore where the edges of the butterfly touch.