I had a similar shifting problem on my '71 280SL. When coming up a very steep incline, the gear shifter would not lock into position 2,
it kept slipping and for about 7 to 10 seconds as I tried to accelerate, the car was revving and almost hunting for the next gear until I moved the shifter back to position 3 and it would then engage and move off normally.
I thought I had a serious transmission problem...
I replaced the Xmission mount and also replaced the filter & fluid- there was a certain amount of fine debris in the Xmission filter pan.
Once I had done this work, the problem was immediately and completely solved.
On a very steep incline the gear shifter now stays in position 2 and the car accelerates normally. I've also noticed very much smoother auto gear shifts in normal drive mode.
A couple of things to check:
Have you checked your transmission mount?
I assume you also drained the fluid out of the torque converter?
Does your filter have the u-shaped rubber gasket?
Also make sure that you have the filter pointing in the right direction (see pic below)
Here's a thread I posted with pics:
https://www.sl113.org/forums/index.php?topic=25341.msg181334#msg181334Best,
Mike