automatic transmission slips when cold

1971 Mercedes 280s began slipping badly from 2nd to 3rd gear when cold. However, only after a turn, not when going straight ahead:confused:
Once warmed up, there is no problem. Not even a slight hesitation or whatever.
Changed fluid and filter (fluid was fine, no deposits in pan)
Problem got worse.
Added can of automatic transmission treatment.
No change.
Drained fluid (came out like new) and put in Dexron VI.
Problem solved :runaround:
After a couple of months, the problem is back although not as bad as it was.
It isn't the vacuum modulator.

I am especially puzzled by the fact that it only slips after taking a turn.
Why would that be? And what do you think could be done about it apart from a complete rebuild?

Are you certain the issue is with the transmission versus a problem with the rear differential? Do you know if the car has some form or a limited slip differential? Have you inspected the brakes to see if you have one that is not releasing, since presumably you're braking going in to most turns? A stuck brake could load the drivetrain enough to give the impression of a slipping transmission. With the age of the car the brake hoses could certainly be past due time to replace.

What a sticking brake would not explain is why the issue seemed to go away for some time after changing the transmission fluid....

-Rod

All the brakes are new. Yes I am certain it is a problem with the transmission.
Also I have to clarify that when the problem occurs, I am always driving away from a stop, since this type of transmission generally does not shift below 3rd gear when taking a corner. And it only occurs when the car is cold..