gotta weigh in on this, I agree with with suginami, don't throw any more parts at this until you check the condition of the ignition wiring harness, I guarantee that you will find brittle, flaking insulation.
We briefly had a 94 E320 with this defect, and now I see that our 95 (with 108k) suffers from the same (although still running well).
No help from MBUSA on a mass defect afflicting '93-95 M104 and the V8's as well.
1995 MB E320 Wagon
2001 BMW 530i
2007 MINI Cooper S (!)