Cranks no start

This vehicle will run fine when it is running and two days later it will not start. It would crank but not start. I figured maybe the battery was too weak. I bought a new battery put it in and boom it started. Which really did not make too much sense because it was cranking with the old battery but it worked. However, twenty four hours later it would crank but not start. I pulled the cable from the battery to see if I re-set the computer it may start again and that did not work? Two mechanics cannot figure it out. Including a Nissan dealer which wants to try the ECM but they said we can start there. I am not willing to throw part after part at a vehicle. These dealerships want you to pay for ten fixes you do not need to find the one you do. Not interested! Any answers would be appreciated!