Macbook pro 2012 start lag and have bugs after shutdown or restast

[fastfly]

I have a macbook pro 2012 with 10 GB ram which is working great but one day I restart it and it start lagging a lot the dock became black with a white boundary and the menu bar is also become black which seems to have transparent, the animation become laggy. I try to fix this and reinstall mac os and it work ok but again I restart it the problem starts, the only way I am able to use it normally is resetting it and reinstall macOS again and again. But it takes a lot of time to reinstall so Please Help me