The car is a 1993 E320 with 75K miles. At random times the engine/transmission goes nuts and will not shift out of second gear. This leaves the car with a almost total loss of power (floor it and you'll get up to a wonderful 15 mph). The car has entered this state while driving down the interstate (70 to 15 with no warning and surrounding traffic - fun), after being stopped at an intersection, and even after sitting in a parking lot for an hour.
Repeated trips to multiple dealers have resulted in zero solutions. The problem does seem to surface more frequently in warm weather, and only after the car has been recently driven (i.e., never first thing in the morning).
We've taken the car to numerous dealers on different occasions (even once while the it was suffering the symptoms), but none have been able to isolate the problem.