Ispinwrenches

Hey guys, I have a 93 300E with a 2.8. The engine harness was toast and it took out the overload protection relay and throttle body. I replaced the engine harness,coolant sensors ,throttle body , overload protection relay and maf sensor.I started and ran beautiful for 3 months and then it started stumbling and had a start/stall. I have double checked all my work and even tried a used computer on it with the same results. It has 12v to the maf and I traced it down to that when I disconnect the bl/grn power wire to the oxygen sensor the vehicle will start and run normal. If I plug the heater wire back into the O2 it goes back to start/stall.I checked the voltage on the wire and it has 12v. I replaced the O2 sensor with the same outcome..this absolutely baffles me. Checking the circuit, it's isolated to the ecm. Has anyone ever come across a situation like this?