Dell Dimension C521 freezes randomly at startup.

I have a Dell Dimension C521. The computer will freeze up randomly when I first turn it on at the Dell loading screen. The bar will load half way and it just freezes there. I have to manually turn the computer off and turn it on again. It usually works fine after that. It’s been doing that a lot lately, maybe 50% of the time.
I ran Diagnostic tests and they all passed. Anyone have any idea what the issue may be?