ML 320 Transmission Slips Out of Gear -- Very Intermitantly
 

Help! 1999 ML 320 with 104,000 miles. Periodically slips out of gear on hwy. Sometimes can drive up to 300 miles at a steady 80 mph, with no problem, and then transmission slips out of gear. Sometimes I can put it into neutral, put it back in drive, and keep going. Other times it seems to go into limp home mode. I then have to park it for 5 minutes and it is fixed for a while. Seem to happen more when the outside temperature is over 75f. No other problems. Other than a few times on long highway trips, the engine and transmission run smoothly. Have had the transmission fluid and filter changed, a new mass airflow indicator installed (in Mexico. Long story.), and now they are trying to replace an ignition coil. The last fix is based on the most recent error codes. My independent / competent mechanic is not going to charge me anything else until this is fixed. He said we are in a guessing game.

I had a few similar things happen with mine. I changed the fluid and filter and also overfilled it on purpose. I'm not sure which solved the problem, but maybe try adding another 1/2-1qt and see what it does.
Another thought based on your symptoms is heat. Maybe a trans cooler might be a good idea.

Vehicle already has a trans cooler built into the radiator. Please do not overfill on purpose, it is not a good idea.

Possibilities, low on transmission fluid. It its full, the transmission has an internal problem. What codes did your tech pull?