Replace Nvidia 8400GS on Vaio SZ650N

Hello Forum, this is my first post here. I have been perusing the site for help on fixing my VGN-Sz650N. The main problem:



Upon switching to "Speed" mode, the GPU freaks out and shows green lines and random text characters while booting into windows. But once booted into windows, the screen functions correctly but the fan whirrs to 100%. I read that it is the Nvidia 8400gs's problem, but is there any way to fix/replace it? If so, where can I buy another GPU?



Would a heat gun fix it?



Thanks!



P.S. Is the only difference between Speed mode and Stamina mode the enabling of the Nvidia chip in the previous?