Thanks for those who helped with suggestions about my erratic shifting.

As it turns out, it was indeed the kickdown cable. I took one quick look at it and it was obvious that I'd find the culprit. It was a forehead-slapping moment because I thought I had checked it. I've got a 88 and an 89 so it's sometimes difficult to remember what I'd done to which car. I know, I know, I should write everything down.

So here's the situation: there is a lot of slack in the kickdown cable where it comes out on top of the throttle body. It looked initially like it might have just been disconnected, but unfortunately it is still connected and there's just a lot of slack, deflecting about an inch or so at idle. If a try to stuff the cable back in to the housing, it goes with some effort. But the problem just happens again. There doesn't seem to be anything wrong on this end of things, so apparently the problem is at the other end. Unfortunately the kickdown cable is one thing the Bentley manual isn't all that helpful about in terms of diagnosis. If I'm getting this much slack in the cable, what is likely going on at the other end? Do I need to just replace the cable, or is there some other fix I could look to do?