Possible ecm problem?
 

I have a 2005 Pacifica AWD touring with the 3.5 engine. About 6 weeks ago I was driving it and the engine completely shut off, still have the radio and lights and everything, but no power to the engine. We took it to our local Dodge/Chrysler dealer and they replaced a bad ground. We drove the car for a couple days and it started bucking while accelerating, come to find out, it was a bad alternator, so we replaced that as well. Have been driving the car for a couple weeks now with no problems, until 2 days ago, when the engine completely died again. Still have all the lights, radio, and everything else. Can drive the car for about 5 minutes and it dies again. System immobilizer also comes up on the instrument panel. Any ideas what could be causing this issue?? Thanks in advance for any help.