About your start up after the overnight pressure test with pressure drop. Any white smoke out the tailpipe? Of course if your driving a mopar classic I'm sure you know where I'm going with this.
Did your antifreeze fluid consumption/loss rate increase during the period of time after pressure test and the running unstable period? Motor ever been overheated or run on the hot side, but still in limits? No need to reply just food for thought in the the even you haven't gone down that road already.