Remember that the New Messages feature that people often look at to see if there are new messages doesn't necessarily show your new message for very long and it could easily be missed. It is one of the failings of this message board software.
I had a B+ running flawlessly for months, until I seemed to develop a hardware issue. No problem, got a Pi2, put all the same stuff on it, had a bit of trouble working out the GPIO stuff, and now it continues to crash within 24 hours to over a week.
If the other one developed a hardware issue and you replaced it and you still have problems, maybe the reason for the problem is outside of the pi boards. If this is being used in a project that matters, I mean you've got a budget for parts, I would definitely have a supply of everything for swapping out. I tell people to pretend like they are Noah and get two of everything. A power supply could heat up and go sideways and even trash the pi boards themselves, who knows.
Did you set up a script to ping the pi from outside and keep track of how long it responds? If it is on wifi, the problem is often that something is going to sleep even if you've told everything you know about you don't want that. I would set up another pi and make sure it is solid and then swap it into the production unit's place.
If it still is crashing, does it crash if you aren't running the software you wrote to do your project? You basically need to cut things up to find out where the problem is to move forward.