el capitan?

[Dee Dee Sommers]

OT:
So my laptop has been occasionally spontaneously restarting itself since I updated to the current Yosemite update: 10.10.5.

The problem is that this is a mission-critical production system.  The spontaneous restarts are NOT cool.
I took the computer in to the Apple Store last week and they ran extensive diagnostics on it.  Everything passed.  Finally, they suggested we wipe the hard drive.  That’s what we did.  So when I brought the system home, it was as a brand new computer.  Instead of restoring from my backup, I have been doing a clean install of all the apps and at this point have not put very many of the user documents back onto the computer.  

And then this evening, it did it again: spontaneous restart.

So at this point I am thinking it must be OS-related.  I don’t *think* I can go backwards on my OS, can I?  If not, is my only option to upgrade to el capitan?

How bad is el capitan?

Thanks for any insights,
Dee Dee