Laptop graphic display issues.

Hello,
when i turn my computer on sometimes it wont display on the monitor but the computer always comes on and sometimes it will display but the graphics are laggy then it beeps once and shuts off or if it is alot for the computer to load it just shuts off but like i said it only displays every now and then on the monitor sometimes it takes 15-20 mins before it even displays anything please help its a new motherboard also..