Similar thing happend to me in 2019. My 84 300D starting flaring abruptly in between all gears. It was very undriveable it would just slip in all gears!. Changed the fluid and filter. Nothing. Read a post on here someone suggested dump a bottle of Trans x (blue bottle) in transmission. It's supposed to clean varnish in a valve body. Anyways I did that. Drove around for like 15 minutes around the block miraculously every gear was shifting like BuTTter!!! It brought so much life in a transmission I was going to throw away. However their were consequences my cooler line started leaking leaking a strawberry mixture of a color. It ended up being coolant and tyranny mixing. It was totally worth it though