Why does my phone keeps restarting after balck bars again and again?

After an update my phone started to show black bars on screen randomly and then immediately shutting off and then restarting. I've tried reflashing the phone and even downgraded 2 updates and then updated it to the latest version, but the problem persists. The thing I've noticed is that this rebooting doesn't happen when I'm in the bootscreen, update screen, download mode, or factory resetting. This only happens when the OS kicks in. Please help.