installed newly rebuilt trans and still have the same problem.

Check that the modulator valve is getting vacuum and is correctly adjusted. The other question is, unfortunately "Who rebuilt the transmission?" I had massive problems with a W126 transmission and the ultimate cause was that the case had distorted and there were internal leaks between it and the valve body. Had to get a case from a wreck. Also look at the B1 and B2 piston.

http://articles.mbz.org/transmission/b2/why/