Bad GPU?

My laptop died, no power, but i had a full geek squad warranty.They replaced the video card and the mobo(i'm guessing heat damage or something). I get it back, it turns on(good), sits on a black screen for like 20 seconds before windows comes up, and it bluescreened within 5 minutes every time. So i get the newest videocard drivers, and it solves the blue screen issue.

>doesnt bluescreen any more
>freezes within 3-20 minutes after i turn it back on, cant move the mouse or anything
>sometimes doesnt even get to the windows login screen
>sometimes a black screen will pop up after it freezes

I need to hold the power button and just turn it back on after. I'm in safe mode now because its just too annoying.

What can I do to try and fix this? I could just bring it back to best buy/geek squad but it took them two weeks to ship it out/fix it/get it back to me and its messed up still. I'd at least like to have a diagnosis to tell them, if i gotta get something replaced.