mac with question mark folder

I experienced the dreaded question mark folder on startup. I first thought it was the hard drive. After replacing it with a new drive it booted fine, but next day it was back to the question mark. So I replaced the cable, installed the original drive and it worked fine. Next day, yup you guessed it, back to question mark. I booted it from the new drive as external and boots up fine. I have run repair on both drives and both come back OK. I have run verify and repaired a few permissions that needed it. Put drive back in, worked fine. next day back to question mark.... So what could be the problem? motherboard? battery? I also reset NVRAM, but no change. This is a mid 2012 13 in Macbook.