Early W123 300D with odd transmission issue

[Kadassa]

Hey guys I have a 1977 300D with a bit of an odd problem Every now and then it tends to burp the transmission fluid out of the dipstick tube while at speed to the point where it will start to slip. Keep a gallon of Trans fluid in the trunk and usually just throw it in neutral and do the normal hot fill procedure. It dosen't do it all the time..and dosen't matter if its hot or cold. This is something I want to figure out pretty quick as having tranny fluid burping out onto a hot exhaust manifold dosen't sit well with me

A bit of history on this car

I bought it from the son of a man that had a repair shop back in the 1980s and around 1989-90 they put a new engine in this car, customer decided not to pay it and instead brought him the title...and so the shop owner drove it home and parked it in his back yard where is sat untouched until 2013 where I bought it for my dad...[whom wouldn't leave me alone about my W115 300D I had at the time] he drove it for a year and a half like that until I ended up with it Engine is super strong and runs perfectly, the shift into 3-4 is a little early but I know thats adjustable and it needs shifter bushings which I have coming in the mail and should be installed next weekend along with a trans filter kit. I know he's never changed the filter in the trans...and it did sit for 23 years. Any info would be helpful! This transmission is not vacuum controlled and has the bowden rod setup I have dealt with quite a few of the old w123-W126 diesels back when I worked in a junk yard so I do know a bit about them..just never ran across this before....I do know that the rubber head on the dipstick is pretty rotten and I don't recall seeing the type of head its got on it so I did order a new dipstick...just in case the guy that put the engine in put the wrong dipstick in it....