Use safe mode, and it will do one round of "Repair Disk" then come up with minimal extensions loaded. At that point, it's all Apple software. If it flickers then, it has a Hardware problem.
To invoke Safe Mode, hold down the Shift key as you restart. It takes several minutes. The login screen will say "Safe Mode" in RED, and you will need your User_id and password to proceed, even if you normally auto-login.