Get a set of gages on it and check the pressure.
The previous post gave a pretty inclusive list of possible failures.
If it comes back on without intervention when the rpms drop, I'd suspect low freon pressure on the suction side.
If not, it's likely a control issue.
The factory A/C manual contains a comprehensive diagnosis procedure, and that's really where you need to be. Don't just start throwing parts at this problem. Verify pressure before doing any replacement.
Now, if you had access to another W126, you could swap out suspect stuff.....
Jim