Macpro sometimes won't boot/won't login

Hi everyone, first time here!

As of yesterday my macpro sometimes wont boot. When it does boot i can get to the login screen, i type in my password, the loading bar gets halfway there and my mac shuts down. It's really weird, i always hear the startup chime, but sometimes i get a screen, sometimes i dont..

One hour ago i got into my computer, i ran disk utility, ccleaner but no errors or whatsoever.. Now i got to the login screen twice but couldnt login!

Anyone?

Thanks!