What component failed in my laptop?

I have an xps 15 9550 that I've owned fkr about 4 years now. I am having a problem where the laptop boots normally, but after being on for about 2 minutes the screen starts becoming distorted, first with aggressive screen tearing then starts to become unreadable until it becomes a grid or rows of solid coloured sections. I'm assuming this is a hardware problem, and have already tried disconnecting, cleaning and reconnecting the display cable. I'm wondering if this is a display or gpu problem. I'd purchase a new display if that is the issue, but don't want to be out $300 and still have the same problem. To confirm, the laptop is fully functional for a few minutes after rebooting, until the display slowly glitches and then completely freezes with random colours. I can connect to an external monitor and use my laptop there. Thanks