ecu bad? 1986 16 valve

I need some advice on my ECU. the car starts fine and idles smooth. the on off ratio at pin 3 of the diagnostic connector reads 65%. When it warms up the needle goes down to 40% but after several minutes it slowly drifts upward stopping at about 70% on the meter, at that time the idle speed varies slightly, and after about a minute the needle slowly drifts back to 40% and the idle smooths out again. this repeats while the car is running. I disconnected the O2 sensor and grounded the signal line to the ECU the needle went to about 98%, I then put an AA drycell on the same place (steady 1.5 v) and to my suprise the needle started drifting down but it stopped at 40% only starting to drift back up to 70% (I thought it should go to about 10% and stay). the results are exactly the same as when the O2 sensor was connected. Is there anything else that could cause such a problem? the ECU is expensive and I am not sure if I am missing something