let it idle & bump/jostle all the ECU/pickup wires & see if you can get it to die. & next time it wont start, pull the yellow wire off of the starter relay to disable the starter & have a helper hold the key to crank & you with your meter see what ain't being fed then have him release the key & pull a batt cable & with your meter on ohms check the pickup for continuity at the ECU. The fact that changing out the ECU makes it OK tells me the prob might be in that connector but I ain't 100% sure.