1993 Ford Escort Momentary Pause/Hesitation When Under Load

1.9 L SOHC motor, 5-speed stick

Replaced idle air control valve, spark plugs and wires, fuel filter, air filter and O2 sensor.

Have stumbling/fumbling/hiccup/bogging of engine under low RPM torque loads, such as pulling away from a light, or low RPM torquing past another car -- does not happen when revving/winding motor up, only when stepping deeper into the gas pedal without downshifting. Once engine "clears it throat" and RPM comes up a bit (2500?) Motor pulls smoothly -- no problem revving motor up to redline, gear to gear -- pulls fine when flat-footing. Only stumbly when down low and grunting a bit, and not all of the time -- gets worse and more frequent when car is warmed up to operating temp -- least noticeable when cold. Starts fine, warm or hot, idles smooth, cruises under light throttle at highway speed without hiccups -- climb a steep highway hill, and, depending upon how far I am depressing gas pedal, may buck/hiccup/stumble a bit -- depends upon RPM, road speed and steepness of hill. Problem is ALWAYS apparent when engine under a lower RPM torque load, not when revving. Have heard some occasional light pinging in mid-range RPM as engine climbs out of stumble and starts to rev up.

Plugs were all burning gray/white. 19" of vacuum, needle steady at idle, 3K miles per quart of oil. No "check-engine" light glowing. Engine temp normal, engine electric fan is functioning correctly, radiator fluid level normal, no leaks or loss of antifreeze.

Thanks for any assistance you might provide. Great car in good shape, despite age, with outstanding gas mileage and reliability.