imac freezes tried safe mode keeps shutting dow

Please try resetting the NVRAM per the directions in How to reset NVRAM on your Mac. If this does not work, disconnect all external peripherals (one of them may be the cause of the problem) and then try resetting the NVRAM again and restarting in Safe Mode. If the same issue occurs, then you may have a hardware failure and you will need to take the computer to your local Apple Store or independent Apple Authorized Service Provider to be professionally diagnosed. Then you can choose the direction you want to take.

Try doing the resets again with all external peripherals disconnected. If the problem goes away that points the finger at one of the peripheral and then it's a matter of trial and error to find the offending device.

If the computer still has the problem with no peripherals connected to it then that points the finger at internal hardware meaning a trip to your local Apple Store or Apple Authorized Service Provider to be professionally diagnosed and repaired.