1992 Chevy Lumina Transmission downshifts rough

After you revive yourself on the subject of how many miles are on this car, I'll give you a short history. This car has been well taken care of and still runs like new. I change the transmission fluid about every 10k to 30k miles. It is the original tranny with no repairs ever. It down shifts roughly, almost with a kick usually from 3rd to second or second to first. It's been doing that periodically for the last 200,000 miles. Changing the fluid did not fix the problem. The most obvious answer is probably it needs some repair after 380,000 mile, however, I am skeptical. Other than the rough downshift, it has operated flawlessly all these years and still does. Could it need a minor adjustment to the shifting belt(s)? Or a sticking solenoid? It does this about 50% of the time. Otherwise it runs flawless.