I have a 99 ML 320 and had to have a transmission rebuild at 184K miles in Jan 2007.
My symptoms were similar to what you are describing. It slipped a couple of times over the course of a week or so. Then finally had a catastrophic failure accelrating up a slight rise getting on an interstate.
Dealer quoted between $5K and $6K to replace. I found an indy who rebuilt it for $2500 with a one year warranty. So far so good (knocking on wood).
Read this thread detailing my experience
http://www.peachparts.com/shopforum/showthread.php?t=175146