Alex's site has the best info for troubleshooting. However it assumes that "everything else" is within specification. The air flow sensor may be out of adjustment, the ECU could be acting up, a bad knock, coolant or thermo time sensor. All of these can cause similar problems. If you have a Bentley (preferred) or Haynes (not as bad as everyone thinks it is) and a multi-meter, use the troubleshooting steps within the fuel system to try to find the problem. You mentioned that you have access to a donor car, so changing the ECU and sensors for troubleshooting can be an option to make troubleshooting easier. All in all however, sounds like a classic vaccuum leak to me. Try the carb cleaner spray test.
Rich Blake
