Gateway laptop screen turn grey after or sometime while booting

Hello,
I have gateway Md2614u for almost 5 yrs now. Great laptop! Loved it and no major problem till now.

Recently, the laptop screen turned grey, it not complete black. In prior days, the screen would go blank after it boots up and everything loads up. The screen would go grey after an hour or so. But now it wont even boot completely. It would go far till the window sign comes up and then goes blank. (There is no cursor blinking)

If I dont use it for sometime, and then power on it would boot completly but after using for 10min or so it goes blank again. One thing I have noticed when this happens, when it is booting or when it is working normal the hard drive light is constantly blinking. But as soon as the screen goes blank the light goes off.

I tried replacing harddrive but it would not let the windows boot files to get copied to the new hard drive. ( it would go blank in the middle of the process). I tried my old hard drive to another laptop and it works fine.
Also, I have already tried replacing battery and turning on without battery( only pluging the power cable. Already did the 30 second holding of power button as well.

I cannot figure out the problem. Can any one help me out please?

Thank you