I assume it's a dead/dying gpu. It's not heat because it comes up this way from a cold boot and it's not software because it does this booting from multiple drives as well as a USB install drive. I also get the same behavior on internal laptop display and external monitor.?
I've seen that kind of issue when a GPU is shorting out as well. More than likely you have a dying graphics card. Is it onboard or a PCI-E card?
+Kevin Medeiros unfortunately, its the on-board radeon 1600 in my old, old macbook pro so there is no replacing it.
Oh shit, yeah that's not good 🙁 You'd have to swap out the whole motherboard in that case. Yeah, that's never a good sign my friend. You could crack it open and make sure all the solder points are intact, but other than replacing that chip or board you're going to be SOL.
+Kevin Medeiros This old machine is not worth spending money on. I'm parting it out instead