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.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles