As cars age probably one of the most common problems is connectors. This is especially true of low voltage connectors that are part of the various electronic control systems.

Remove the connector from the ECU and carefully check all the pins and receptacles with a magnifying glass. Look for bent pins and corrosion. Scrape off any corrosion with a dental pick or Xacto knife. Then clean everything with contact cleaner.

Sometimes just "cycling" the connectors will solve problems, but a thorough inspection of all engine control system connectors including the ECU and all sensors and actuators will usually be beneficial.

Duke