Toyota Repair: 1991 Toyota Camry stumbling

Question
I have been asked to look at a 1991 Toyota Camry for a friend. It is the California version, with two oxygen sensors, and it is the 4 cyl engine. The car sometimes runs ok when cold, but as soon as it warms up at all, it starts stumbling and coughing as though it is running out of fuel. The car will idle fine, but under load, it bogs down. I am not getting a check engine light. The spark plugs, distributor and cap are new. Fuel filter and air filter have been changed. I can locate no vacuum leak. The throttle position sensor appears to check out fine.(It is a switch type, and seems to work.) I did unplug the oxygen sensors, one at a time, then both, and the car did not run differently. I think thtey may be bad, but did the O2 sensors in this old of a car adjust fuel trim? What did they do? The MAF is mechanical on this car, but if I unplug it, it runs really bad and I get a check engine light, so I think it working.
Any guidance to can give for things to try is appreciated.

Answer
This year still used the OBD 1 system although the front O2 sensor sent a feedback signal to the computer it did not adjust fuel ratio very much, in other words it's parameters are very narrow compared to todays air/fuel ratio sensors. I think I would check to make sure that the moveable door in the air flow meter moves freely all the way open and that there are no leaks in the intake hoses between the throttle body and the air filter.