If the problem always occurs, replacing the ECM will tell if it is the problem. If it is intermittent, it's a very expensive part to throw at the car and you still won't be sure if it helped.

If your dealer is honest, they will remove the new ECM if it doesn't help. You can't buy one to test, as they won't let you return it, but they could put it back in stock if they're doing the work.