HELP--1990 audi 80 not a quatro 4cyl automatic

[mskwara]

Erratic idling--Changed the fuel pump(noisy), injector seals and seats, 
fuel filter, throttle body gasket,plugs, wires and rotor, fuel 
distribution unit, idle stabiliser valve, idle and wide open throttle 
switches checked ok, control pressure actuator cleaned and replaced, 
temperature sender for  ECU, oxygen sensor,  compression good,  intake 
gasket.  Some parts used off an '89 parts car.  Can you tell me where 
the ECU is located ?  Car will run good for a period of time and then 
will go too lean or too rich upon resetting with 3 mm wrench.