Codes 23, 24 and 12, sluggish acceleration

1987 Nissan D21 pickup, 2.4 liter, Z24i engine automatic with push button overdrive. 267,000 km. When I purchased the vehicle about 3 months ago, only issues I noticed were the engine idled a little high, the Park light indicator on the dash display always stayed on regardless of what gear I was in (didn't really feel this was an issue so left alone), and the steady bearing on the driveshaft was shot so I replaced it. Gas mileage was fair (about 22 miles/gallon CDN) but I thought it could be better.

Then driving to work one day, the truck accelerated very sluggishly and I could barely get it to go 100km/hour. First, I changed the air filter, fuel filter, and plugs with no improvement. I checked for ECM codes and found code 23 and 24 which would reappear even after disconnecting battery in an attempt to reset it. I replaced the O2 sensor and PCV valve. I swapped out the thermostatic air cleaner vacuum motor, air temperature sensor, mass air flow sensor, TPS, idle control valve, AB valve, and fast idle control solenoid with ones I obtained from a local auto wrecker. The truck would now accelerate over 100km/hour but I had an irregular idle and hesitation while accelerating but once I drove about 10 blocks (basically, once it warmed up) it idled and accelerated fine. I also inspected the EGR valve, cleaned the reed valve, and ensured all hoses were intact with no air leaks or blockages.

Then about 1 week later, I noticed I forgot to include a round rubber gasket when installing the replacement mass air flow sensor. So I removed the sensor, inserted the rubber gasket, and re-installed it. Now, the truck wouldn't start. I tried for about 15 minutes and finally got it running but it ran extremely rough, hesitated, wouldn't idle and smoked. I finally gave up and took it to Nissan. They swapped out the injectors claiming one was dumping fuel and the other was spraying irregularly. They tested continuity for shorts and grounds. They also adjusted the idle and TPS setting. I picked up the truck and it sounded better but I still needed to pump the pedal to get it started and it would not hold an idle. It was still very sluggish while accelerating and would even cough when it tried to gear up. Code 23 and 24 still appear and now code 12 appeared as well. Nissan said it's either the ECM or the Mass air flow sensor but said since nothing changed when trying each of the mass air flow sensors I had provided, they recommended replacing the ECM. So I purchased and installed a re-manufactured ECM from Bluestreak the other day and while attempting to start the truck, it turned over for about 10 seconds and was just about to fire up when I noticed fluid coming out of the ECM and smoke was emitting from it as well. Very frustrated, I checked for codes and found code 23 and code 24 again.

Nissan says its a bad ECM but why are these same codes showing on the new ECM as well after all I have done? I am thinking a short is coming back to the computer from a connector, bad wire, relay or sensor but why didn't Nissan detect this from the continuity tests? What can I do? I have spent over $1,000.00 on this truck already so would greatly appreciate any feed back. Thanks very much.