ddec 4 goes dead randomly

Bad ECM grounds and poor 12 volt feed to the ECM can cause issues. I've even seen the pins in the harness connections spread over time and lose contact. I would do what everyone else has suggested and ensure that all your wiring to the ECM is good. I would say it's unlikely your ECM has failed since it tested ok. I've been pulling wrenches for 8 years and I've only ever seen 2 ECM's that had failed. 1 because it had been hit and another that had failed due to not being properly sealed, which allowed water in.