Maybe the new ECM is faulty? Maybe they replaced the LPG injector driver (separate unit IIRC). Maybe that has a new part number too.

I vaguely remember something odd with the dual fuel switch too. I don't remember exactly what it is but it has a middle or "neutral" position and I think you have to have it in the right position before you shut it off, or on, ah I can't remember at the moment.