Why HP G61 Laptop Screen Blinks?

I have an HP G61 Laptop it boots up just fine has no trouble. But if I’m using it for 3-4 hours the screen starts to blink. I reinstalled the video drivers it still blinks. I hooked it up to an external monitor the external does not blink does this mean I will need to change the screen? Or could it just be a cable? Any ideas thank you for your help.