I agree with the previous replies that your transmission sounds like it’s on it’s last legs, with hardened-aged seals being the likely problem.
Since you have nothing to lose, you may want to try a can of Berryman’s Chemtool. A transmission expert who used to frequent this site (transman618) used it often in aged transmissions to soften up hardened-aged seals. It’s not a product built for transmission use, but it’s reputed to help.
Here’s one thread where he discusses it.
There are more. Just search this site for old threads about Berryman’s Chemtool and/or threads from transman618. Consider it a bandaid, especially where you have nothing to lose.