I am having identical problems with my 300TE-24v. I also get hard upshifts when hot. My problems started when I had water in the trans fluid - it was OK with the water in but started after I flushed & refilled the fluid (several times).
I've been driving it for about 9000 miles since & it hasn't got any worse.
I've been putting off a rebuild but will have to do it soon - my wife refuses to drive the car.
Please let us know the solution to your problem - especially if it doesn't involve taking the trans out of the car again!!
90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)