looks like there is no real best place for 1st gen for this. I think you should go to 355nation. They are more into 1st gen.
to your original post. it seems like it must be an electrical not mechanical transmission problem first off. Do you have a scan tool? If so I would take a close look at the map sensor and throttle position. Other than the ECM just simply goofing up those are the only things I can think of that could cause something like that. But, with that being said it would be odd that if it was one of those that you don't have other drivability problems too. So, my GUESS would be the ECM because there is no check engine light and a quick power off/on fixes it. Of course it could be something in the trans itself like a sticking solenoid but I just don't know enough about it to tell you what. Sorry, this is as weird of a problem as it gets. Check over at 355nation, you might get lucky and find a trans expert over there or at least other ideas. Good luck!
Now that I think about it, if you are so inclined, check the wiring plug at the trans just to be sure its not corroded or loose or whatever. A good visual inspection is the best start to any problem anyway. I also wonder if the trans selector switch is ok. Does it only do it if you manually put it into 3rd then back? If so thats a possibility. But then again the key cycle fixes it so probably not. just goofy....sheeet
I'm curious. If you just drive it and don't mess with the trans selector (leave it in D), does it downshift and everything correctly? Does it run right other than this? Any idle or other problems?