If the terminals of the ECU were coroded, then the chances for the termanals in the plug connectors on the wiring harness connected to the ECU could be dirty/ damaged. i suggest you unplug the wiring harness connectors and clean them thoroughly. You can bend the lugs inside the connectors so they fit tighter onto the ECU terminals.
Hello, I'm an ASE Master Technician and I would be glad to answer any questions you may have. Please refrain from rating me until you are satisfied. Feel free to continue our conversation for as long as you need.My opinion is as follows. The fact that your tachometer is now stable suggests you correctly identified and repaired your problem. Anything that prohibits a proper connection would essentially cause the circuit to go open which would interrupt the circuit causing problems. In your case, while it sounds like you had a connection on the tach feedback, it was a poor connection causing the fluctuation. It would also be good to check the terminal fit/drag. In addition to contamination causing a poor connection so too will a poorly fitting terminal. You would need a pin(sewing needles of various diameters may work) the same size of the pins in the ecu and associated connectors along the circuits path. I have special tools for this which have terminal pins of varying diameters and shapes which you simply insert into the female terminals then pull out. What you are looking for is sufficient drag on the pin to indicate the female terminal is good and tight. If the pin pulls out easily with little or no drag then the female terminal is too loose and would need to be either repaired or replaced. You need to be very careful not to use a pin that is too large as it will spread the female terminal open wider making your problem worse. I'd also make sure that all of your wired terminals are pushed all the way forward in the connectors, sometimes they can back themselves out over time .I honestly believe you fixed your problem and the proof is in the pudding(it works). I've seen a lot of problems resolve themselves simply by unplugging the connector and plugging it back in. Sometimes a pin has backed out a little over time or there is corrosion which is wiped clean when you unplug and replug.If there were an issue with a circuit board etc then I suspect it would still be acting up even after cleaning up the connections. If it happens to return then I would start by checking the connectors pin fit. Make sure they're tight and staying put inside the plastic connector(give them a little tug). Then I would check any pins soldered to a circuit board(i.e. ecu). Sometimes a poor sporadic connection comes from one of these pins starting to break free from the circuit board at the solder joint. You can usually only tell by looking at the solder joint while wiggling(gently) the pins.If you need me to elaborate on anything or if you have any other questions, please don't hesitate to ask. Thanks.