Transmission will not up shift

Newer models use a cable instead of that linkage, and those are prone to sticking in the part throttle position. Since you obviously understand the purpose and operation of the linkage, I suspect you are on the right track with the varnish build-up. I ran into this on my 1988 Grand Voyager that used the front-wheel-drive version of the 904. I drove this van for many years, including fifteen years dragging a tandem axle enclosed trailer that is bigger and heavier than that van, to old car show swap meets. It had one fluid and filter change in its life. In all those years, I never once hit wide-open-throttle, even when pulling the trailer, so the throttle valve never got exercised throughout its full range. I suspect varnish was all I had left in there for transmission fluid, and it had coated the bore where the valve had not been running.

While experimenting with an intermittent cruise control problem, I had the bright idea to connect multiple test lights to the servo's terminals so I could see what was happening. Did not stop to realize those valves are turned on by grounding various wires, which is what the test lights did. Come time to head home for the night, when I started the engine, the servo pulled the throttle wide open and held it there until I could scramble around and turn the ignition switch off. Luckily there were no pistons going through the hood, but that is when the shifting problems started. I had to hit roughly thirty mph at low-throttle before it would go into second gear, and about sixty mph to eventually get it into third. It also down-shifted much too early when slowing down. Being the king of procrastination, as with any other "minor" problems, I ignored this one too, and every day after that, the shifting got closer to normal. Within six months of daily driving, there was no longer any hint of delayed up-shifts or early down-shifts. I never used any additive and never took anything apart. It just gradually cleared up on its own, but I was still afraid to hit wide-open-throttle after that.

That slot in the linkage is not needed for its operation. You will see the spring holds it tightly to the shaft that is pushing on it. The concern is if anything were to happen to that linkage, such as binding, rusting, catching, or anything that prevented it from returning, it would also hold the throttle open, which would be a safety concern. With the slot, no matter what happens to the linkage, the throttle can return to idle.

If varnish build-up is the issue, the governor valve could be sticking. That is what tells road speed to the valve body. If it sticks at above about thirty five mph, the torque converter clutch could still be locked up. GM had a common problem with that on their older front-wheel-drive cars in the 1980's. Those also caused stalling when coming to a stop, but they were electrically-controlled. The solution for a lot of people was to simply unplug the connector for the lock-up solenoid so it would never engage. That way it could not stick.

Given there is so few miles on your transmission, I would suspect a speck of dirt is causing the governor valve to stick before I would suspect varnish. It is on the output shaft inside the tail housing. If I remember correctly, you can get to it without removing the transmission. There should be a small plate on the bottom, held on with two screws. Remove that, then you will see the ends of a large snap ring. Remove the tail housing bolts, then that snap ring has to be expanded or squeezed to let a bearing slide out of the housing.