My 1999 ML320 has developed a peculiar problem at 50,000 miles. It started going up a steep grade at high altitude. All of a sudden, the car slowed, then sped back up, then slowed, etc. I took it to the side of the road, turned off for a few minutes, started back up easily, and began to drive again. Same symptoms.

I had the car towed. The dealer had no ideas as to the problem. There were no indications from the computer and no lights on the dash.

It took a couple of weeks for this to happen again, then a few more weeks before the next occurance. During this time, we drove a lot, including some very steep grades, including the one that started the problem.

Finally, two days ago, it happened again. Desparing of making my destination, I turned around and the problem happened on the down slope! After several stops, I got it to a mechanic who made it fail, but has been entirely unable to diagnose the problem.

Again, it slows down, then speeds back up. Shifting down makes the problem worse. Sometimes it slows down to the point of taking the car to a stop. It never stalled until the mechanic had it, but many times it took a minute at idle to get the rpm's to go back up.

Help! Any ideas?