I had an '82 300SD that had similar trans problems. I used the Gunk additive as well with similar results. I was told repeatedly the problem was hard seals. I found that as the trans heated up from driving it got better.
Question: Did you drain the torque converter as well?
Valvoline now has a trans additive that helps "condition" older seals. I would remove enough ATF and add a quart. It might return the flexibilty of the seals.
Additionally, I was encouraged to check-out the vacuum modulator and the lines to it. Have you?
On our (past) 300D this was the problem.
Let us know ....
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)