You are at relatively high high vacuum with a light load at 2000 rpm in fourth gear. That is probably one of the hardest tuning poinst to get right, it is sort of a light coast. You will get a moderate swing in vacuum as you try to maintain the speed, therefore, the base injector map is changing or swinging. At the same time you are in closed loop and the O2 sensors are trying to maintain a fixed a/f ratio - in affect they are slightly fighting each other. When you push the throttle down, you go to a lower, but steadier vacuum, therefore, the injector map is close to steady state and it will not fight the O2 sensors.
On the other hand, as stated earlier, maybe it is as simple as a plug wire, although, probably not a plug. A bad plug is pretty easy to feel or hear.
