Why a transmission would occasionally slip from 2nd to 3rd?
 

It's 600SEL 1992. I was having a slipping problem from the 2nd to the 3rd so I replaced the whole transmission with one imported from Japan. When I installed it it worked perfect, even I noticed more HP. After about one week on using it I made a kick-down so the transmission down-shifted to the 1st, after that I noticed slipping from the 2nd gear to the 3rd gear. Then after 1 more week the slipping problem disappeared and will occasionally comeback again. What could cause such a problem?

I just went through this yesterday. My transmission had recently been rebuilt by a knowledgeable tech at the dealer. It started slipping occasionally on the 2-3 shift only. It would catch, but the rise in RPM was noticeable. The 722.3xx troubleshooting manual says to check the modulator pressure and adjust if necessary. When I took it back to the dealer, the tech pulled the hose off the modulator on the left side of the transmission and transmission fluid dripped out. Apparently, the diaphragm ruptured and transmission fluid (about a half quart) was actually working its way up into the intake manifold.

The modulator cost about $52.00 and I was charged $124 in labor to replace it, since it was not replaced during the rebuild. I think I could have replaced it myself, but the adjustment was something that I wasn't sure of. It seems to work fine now. I don't know if you have a similar transmission, but maybe the modulator pressure just needs to be cranked up a bit.