Anyone out there have/had any experience w/a faulty "engine control module/unit" ? (larger of two boxes that sits in left rear corner against the firewall as you gaze under the hood). Symptoms are: rough to no idle/stalling; higher RPM's/freeway speed ok - as diagnosed and demonstrated by MB repair shop. Is there any way these things can be bench tested and verified faulty and does anybody certify reconditioned or slightly used units? Thanks.