An EF Falcon shouldn't need the throttle position sensor reset, they work on a different system to pre EF's, the EF only has one throttle position sensor and it goes straight to the engine management computer, if there was any problems with this your car would be in limp mode. when in 4th gear with the touque converter locked up your car will be at lower revs than in any other gear normally, so i would be more looking at spark or injectors, it might not be running properly on all cylinders, and seing that you just replaced the coil pack i would suggest checking all your plug leads, they don't like being move around a lot and are easily damaged. If you open your bonnet on a dark night and have a close look all over the plug leads, you should be able to notice if there is any spark jumping from the plug leads, you will see it flash and if so i would replace the lead's which do it, and then see how ya go. if not then check your Plugs. but i would be more thinking the leads would be the problem.
And spark problems can come at all different RPM and loads, i have seem cars miss at all diffent loads and rev's.
And if you ever change something and something else seems wrong than you should always go back and check what you changed first.
Although you have put a new coil on it, i would also check the coil out, i have purchased a stuffed coil before which would cause the car to miss under load, the minute i replaced it with another one the problem was fixed, but i would look at that last after all else fails as it is highly unlikely.