Evolving worsening shifting issues- 1984 300TD
 

1984 300TD 292k miles. Trans was replaced at 277k by the previous owner 3 years ago. Originally she wasn't always starting in first gear, only when I shifted the gear selector in the L setting. I adjust the bowden cable a little and she she would occasionally start in first if I FLOORED it. Then she started being a little confused as to when she would shift into 3rd and 4th gear. Sometimes it would be a hard shift and others nice and smooth (maybe a vacuum modulator issue)? Just recently within the last week she seems to be a complete mess. Sometimes won't shift into 4th gear at all, sometimes while in 4th it'll slide out and start to free rev. I notice when stopped and I shift it into drive you can hear a sound towards the rear of the car like it's trying to engage and then after a few seconds it will.

Trans fluid looks OK. Isn't completely red, but doesn't smell burnt.

It's frustrating since I've fixed A LOT up on the car, runs amazing and she's COMPLETELY rust and rot free and I'd like to sell her, but can't like this :/