[Geoc]

If you know your way around a car, you can also try reflowing the solder on the ECU and TCU boards. Over time, weather and vibrations may have cracked the solder joints, and cause bad connections on the electronic system.

So if you have time, take out the ECU and ECU and take a solder pencil and reflow each connection, taking a multimeter to things like the capacitors and resistors is also could also solve any potential problem shops may have overlooked. Automotive electronic systems from the 90's are very dumb and robust, a lot of malfunctions are usually caused by bad connections from cracked solder joints or dead electronic bits (capacitors most common and resistors also a possibility)