I am having the same sluggish problem with my '88 260e. I have had this problem for months and noted that the car starts and runs absolutely fine for the first 5 minutes until the temp gauge reaches 60 degrees C, then the car gets really sluggish. Pressing on the gas does almost nothing and pulling out in traffic is downright dangerous. When the temp gets up to the normal 80 degrees the car runs fine again.

Based on the symptoms, my problem seems to be related to the engine temperature so I am thinking it might be the coolant temperature sensor. I had an old '75 Volvo with K Jetronic and it had the same problem which turned out to be the warm up regulator. The warm up regulator sat on the top of the head and adjusted the fuel injection control pressure based on the engine temperature. I'm not sure if the Mercedes has one of these or not but if it does that is another possibility.

If anyone figures out this sluggishness problem please post a follow up and I will do the same.