If you have not replaced the engine wiring harness, that would be the first thing I would check. All the cars prior to 1988 had 'defective' harnesses that aged prematurely (after about 10 years) and the insulation started falling off. If yours is looking bad, replace it first, then start fault tracing as the electrical problems can cause all sorts of symptoms.

Other things to check -

If the idle is too low, go through the constant idle speed system - the control valve, hoses, etc. Remove and clean throttle body.

Check for breaks/cracks in intake plumbing

Check engine sensors (too lean when cold or too rich when hot will cause loss of power)

Be sure that all electrical connections in engine compartment are clean and treated with silicone 'tune up grease' (except O2 sensor connection)