I have the same problem, car is 93 300e M104 3.2L w/ 170Kmi. I bought it w/ 150Kmi and no record and no problem except no start turn out it only need new OVP relay which I change to the new one. After that I change all 3 coils, spark plugs, and spark plugs wires (3). About 2000 mi ago, I got the surging idle problem, pulled the code got code 13 on pin 8. Mileage drop from 19 something to 14 mpg. Exhaust smell wierd, black carbon deposit at the tail pipe. Pull the spark plugs, all 6 look black (carbon deposit) seem like it running too rich, fuel not burn completely. Try to find any vacuum leak but found nothing. Took the AMS out and clean it with the AMS cleaner, seem to help a little bit. Change the new air intake temp sensor, nothing change. Recently I read about the ETA wiring, so I design to investigate that. Pull the ETA and splice the wires, bad news. All wires are disintegrated. New one cost $1,300, rebuild around $800 which I don't think I can spend on this old girl. Anyways, I'm in the middle of rewire this ETA now and will let you know if it cure the surging idle problem. BTW, you can check your ETA wire without pulling it out, just peel the shield of ETA. BTW, I did check the number of main engine wiring harness, the number suggested it had been replaced but the lower one is bad (my next project) but I don't think it caused the surging idle.