My 91 Honda Accord EX coupe (automatic) with 170K has begun to run rough between 1500-2000 RPM under load.

I first thought it was a bad cap & rotor so I gave it a full tune up (new NGK plugs, rotor, cap, fuel & air filter, oil change and checked timing), problem continued.

I thought maybe it was the o2 sensor so I replaced it, problem continued.

I checked the ECU for codes and 12 was displayed although check engine light is not illuminated.

I checked the EGR and it seems to be operating correctly but I still replaced, no change. (Note: I used salvage part, looked new)

I then changed the purge cut off solenoid, EGR control solenoid and constant vacuum control (CVC) valve. NO CHANGE! (Note: same as above)

PLEASE HELP!