[W126 Coupe] Transmission advise for an 83' 380SEC

[DH]

My transmission stopped changing to 4th gear (158,000 Miles). It started off with refusing to change up 
when the engine is cold to now not shifting to 4th. Other gears seems to function ok.
What I have done so far:

 1. Changed the fluid+filter including the Torque converter. 
 2. Adjusted the Modulator (hard to soft back to hard shift). 
 3. Replaced the break booster vacuum line due to a leak.
 4. Checked the fluid level.
 
The old fluid looked clean after 30,000miles.
Is there something I can do short of buying a used or a rebuild transmission?
If I were to consider a transmission, can I use one from a late MB *like the 500E*?

Thanking in advance for your help,

Dinesh H
83' 380SEC
85' 190E 2.3
93' 190E 2.6