I am driving a 1993 Mazda 323 with about 88k miles. It had 24k miles on it when I purchased it in 2005. Its transmission was rebuilt 2-3 years ago because 3rd gear was totally worn away.
The car has started downshifting at random times when accelerating to around 20 mph or so. I have never had a random downshift problem at higher speeds. There are periods when it downshifts a lot, then periods when it is totally normal.
I also noticed that on the highway in overdrive for a few hours, the car was not engaging the torque converter. It would however downshift to 3rd like normal when I added more gas going up a hill. Normally if I lay off the gas it will disengage the torque converter, then it will engage after a few seconds when I put my foot back on. But it did not engage, even after I stopped for gas and shut it off, then got back on the highway. It was not downshifting randomly on the city roads during that stop however. After a few hours of highway driving again, suddenly the torque converter engaged and it was engaging normally again.
As the downshifting started happened more frequently, while stopped at a stoplight in drive, I found the car would sometimes roll backwards to varying degrees on hills, sometimes even on the slightest incline that you would not even notice otherwise. I understand having some rollback on modern cars is normal, but it is not normal for this car and it has never happened before. This rolling back while in D also triggers the hold light to flash (the code is always a bad speed sensor). When the car is rolling back, it takes a lot of gas to actually make the car move. The car revs and it take a long time for 1st gear to engage fully, like maybe 4 seconds. 2nd gear is also labored taking a couple of seconds. When moving forward to some degree, the hold light stops flashing. I feel like this action is wearing down the transmission quite a bit. Normally the gears engage very solid (the pump was given maintenance when the transmission was rebuilt).
To try to fix all these issues, the code was followed and the speed sensor was replaced (the whole instrument cluster was replaced with a junkyard one with 75k miles). Nothing else was done. There was not an issue for a few months when I would certainly have expected to see a problem again if that replacement did not address something. But now suddenly the problem has emerged again, progressing in exactly the same way it was seen originally. It either has a hard time engaging the low gears, it downshifts randomly while at a low speed, or the torque converter does not engage in overdrive. And all of these seem related, somehow.