since there aren't any fault codes to read on your 89 (do you have a little black box or IDU mounted on the driver's side strut tower?) I'll guess it's either faulty deteriorated wiring (look for wiring insulation breaking down ør entirely missing) or an intermittent ignition module. Only effective way to test intermittent ignition module is to replace it with a known good one. If problem stops you found it. If the problem wasn't intermittent you could possibly test the voltages to/from the module, but otherwise you're stuck.