Hangs on startup

Hello All,

So we recently upgraded about 20 or so macs to yosemite via clean install, these mac pros are about 6 years old, have about 8-16gb RAM(some have higher some have lower) after upgrading we have various encounters with booting up the Mac, it hangs at about 50% and doesn't move, I cleared the SMC and the PRAM which does the work for about a week but then it will come up eventually,

I also repaired disk permission as well as any driver errors.

Anyone have any other suggestions?

Thank you