It sounds like the obvious sources of the problem have been replaced.
I know there is a problem with defective wiring harness on our models that might be shorting out and causing your problem. I don't know which one though. The other possibility is the OVP, or overvolt protection relay. It is known to be the source of intermittent problems. Do a search on both on this forum, and you will find a wealth of information.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car