Do I understand from your post that you're running without the kickdown cable properly connected at both ends? If so then up-shifting will be horridly retarded, normally even worse than you describe. Otherwise, what you're describing is the symptom of an incorrectly installed or mis-adjusted kickdown cable. The throttle cable should have nothing to do with this issue provided there's a little slack when the throttle is closed.

With the adjuster backed off all the way, there should be a tiny bit of free-play (as little as .01" between the copper ferule and the end of the adjuster) before there is any resistance from the spring loaded pawl in the trans (on the other end of the cable). If not then either the other end of the cable sheath has been pulled out of the trans a bit (rather likely I'm guessing) or this is not the original kickdown cable and the ferule was incorrectly positioned on the braided cable by the last installer.

The 1/8" ferule to end-of-adjuster rule for kickdown adjustment is only half-valid with the factory installed cable or where the last installer took the extra time to position it accurately. In any case, the ferule is really only there to prevent the cable from slipping back into the sheath and should not be used for anything but initial adjustment. Proper kickdown adjustment is achieved by slacking off the cable then plucking it (in a quiet setting) until you can hear and are familiar with the faint thunk as the pawl returns up against its stop. You then adjust the cable tighter until the thunk just disappears and carefully lock the adjuster in place without changing the setting. It may take a couple attempts to achieve this. You can adjust the cable a little tighter for slightly more aggressive shifting or a bit looser for slightly smoother shifting. Make sure the kickdown cable stays in the correct slot in the throttle spindle. It's easy to have it jump over a slot when you're fiddling around.

I hope that addresses your problem.
--
Dave -940's, prev 740/240/140/120 How time flies when you're having fun