Did you added any new RAM sticks recently?
CPUs have a QVL (Quality vendor list) list of approved RAM vendors, make sure your RAM's vendor is one of them.
I have purchased GTA 5 yesterday on steam. After installation was facing "Error: GTA 5 exited unexpectedly" and was proposing options for safeboot with lower graphics option which too didn't yield any positive result. Please assist me in resolving these errors. I would be happy to provide you...