Black screen on HP laptop,Inverter heating up

Hello,

I have a NX 7000 , I have been having an issue that my screen was black, but when I used an external monitor I was fine. At first I thought it was my back light, but someone suggested that before I change the back light, try changing the LCD inverter.

I changed my inverter, and the screen worked instantly. The problem is , it works for about 15 minutes or so until it warms up, then I get that high pitch sound, the screen flickers, then goes black again. If I let it rest and cool down for 10 minutes, and start it back up, it will work great again for 10 minutes, then the same thing happens again.

I thought maybe I had a bad inverter, so I have tried another one and the same thing happens. I can count out the back light, because that works, the inverter works. So what would cause this issue of heating up the inverter?
Thank you!