Hey all, newish Mac user here. I recently acquired a 2010 Mac Pro Eight Core 2.4 with 32gb ram and the original AGI Radeon 5770 1gb GPU. I have hooked it up to several displays each with different cables, and all I can get is a screen with many vertical pink and purple lines. Otherwise the machine seems to be operating normally, Apple chime, all fans spinning properly, HDD chirping away. My initial thought was that the GPU was bad, so I replaced with an identical 5770. This time each monitor that I hook it to reads No display. So I returned that GPU and purchased yet another identical 5770. Same thing, but if I replace it with the original GPU, the pink and purple lines come back. Since then I have tried 2 other GPUs a GeForce GT 710 2gb, and a GeForce GT630 2gb, with the same results, no signal. I should mention that I have tried them in each of the 4 PCIe slots, the original 5770 was in the bottom slot, and I have installed High Sierra on the HDD. This is absolutely maddening as for the life of me I cannot figure anything out with this thing.