You are closing in on it. If engine starts and runs when cold, then stumbles to the point of undriveability when warm you obviously have a temperature related problem. A similar condition on my M117 engine (560SL) was caused by a failed engine temperature sensor. This is the sensor that inputs to the engine control computer. Don't confuse it with the water temperature gauge sensor or the cooling fan sensor.

I would check that sensor using the cold/hot resistance readings given by your tech manual. The sensor is not very expenive ($30 or so as I recall) so if you want to just swap it and hope to get lucky you could try that without being hurt too badly.

Let us know how you fix this problem, it adds to the body of knowledge.