MacBook Pro mid-2012 have to wait 30min between reboots My MacBook Pro mid-2012 has been acting up lately. A year ago I replaced the original hard drive with a Samsung SSD 850 EVO 1TB, and everything was fine. Then the SSD became corrupted and was exchanged under warranty. Since installing the second SSD I've installed a fresh copy of the latest macOS Sierra on a single 1TB partition and have been having issues ever since.

The symptoms are: 


Random fails: When I wake it up from sleep or open the lid I find it with the "grey screen with crossed circle" (Similar to this: https://qph.ec.quoracdn.net/main-qimg-b29f58e707cef6f20a882248f4ee558c). 
Fails to find a bootable volume when rebooted (or even after a shutdown and reboot) within a ~20-ish minute timeframe (boots to the folder icon with a question mark, similar to this: https://i.stack.imgur.com/V5agg.jpg). I have to wait and try to boot it again later. Then it boots as if nothing happened.


What I have done:


Reset the NVRAM. Result: no significant change in behaviour
Reset the SMC. Result: no change in behaviour
Did the Disk Utility trick where I modified the size of my partition, then set it back to a single 1TB partition again, in order to have Disk Utility rewrite the partition map (or something). Result: now it doesn't constantly fails when I close and reopen the lid. Although it may have been the result of the next point.
Changed the energy settings to default (I have a script I run when I setup a new computer, and it includes some non default power management settings). Result: see previous point.


Questions:


Does this seem like a hardware failure or a software failure?
What can I do to be sure either way?
Any suggestions on how I can reliably fix this?


I have several full backups of my data, so I can try virtually anything. I'm determined to find out how to fix this. Perhaps installing a stock version of macOS Sierra and not changing anything at all, and using it like this for a few days could help me figure things out in more detail?
Some Mid-2012 MBPs had an issue with the Hard Drive Flex cable that could cause similar issues (at least as far as the Booting to a Folder with a Question Mark), so I might be worth calling Apple or heading to an Apple Store to have them have a look.

If it is one of those ones, you may get a repair for free for it. I gotta stress though, it isn't ALL mid-2012s, just some of them. Mine wasn't one of them (I had permanent "No Hard Drive Detected" but the tech I saw initially thought it was, hence me knowing anything at all about it).

If it is the cable that is faulty, it didn't cost a lot out of warranty anyway, although if you are getting the odd Prohibitory symbol, then you may want to reformat the drive and reinstall OS X anyway to get rid of any filesystem problems that would still be there beyond hardware