ML 320 Transmission Slips Out of Gear -- Very Intermitantly

[mlandrum78130]

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.