Test your PCI-e cables and make sure they're supplying enough power, and aren't damaged.
Edit: Your R9 280x is listed as being compatible on the MSI websites, which means that your BIOS settings aren't correctly configured to use the R9 280x, your PCI-e cables are damaged, or the PCI-e lane is damaged. I doubt the PCI-e lane is damaged so it's either something to do with your BIOS settings or PSU.