Transmission skipping

Vehicle; 1980 LeBaron (149,000Km); 318 (49,000Km), A904 Tranie (71,500Km).
Generally the car runs great (after new rebuilt carb with ported vacuum,
installed), no rust, no body damage, slight vibration over 80 Km, another
problem for another day.
Problem; Tranie has a hard time shifting from 1st to 2nd if the speed is
increased gradually while going up a STEEP hill . It tries to shift but
just rattles. If the throttle is increased it shifts OK, it just seems to
do it at slow acceleration.
On a flat road, or on a medium steep hill, at any rate of acceleration, it
shifts fine, and you can barely notice the shift. During stop and go
traffic (rush hour?) where the speed is up and down between 15 and 20 Km,
the first dozen or so 1 to 2 transitions are smooth then it starts to get
progressively harsher.
City driving (stop light to stop light) is fine, no problems.
I've recently replaced the filter and fluid (read the other post about fluid
change). There was some sediment on the pan and magnet, but nothing I
haven't seem before.
The problem wasn't evident before the fluid change, but then it wasn't used
in rush hour or on steep roads. The current fluid smells OK, but looks like
blood (red yes, but murky). Used Dexron 3 with Mercon, couldn't find just
Dexron. Yes I checked the level of the fluid and it is almost at the max
mark on the dipstick
Is there an adjustment that needs to be done or is this a tranie rebuild,
again.

Had a 68 Dart with a small 273 V8, same problem. After taking it to a
tranie shop for a free tare down and inspection, then refusing the repair
(they had to reassembly the tranie and give me the car back, in running
condition), it never skipped again, up to when I sold it 18 months later.
They never told me what they did.

Thanks in advance