Assuming it's 1) getting enough power and that 2) it's not overheating I'd just RMA it. It's not worth the hassle of messing with it any further.
It may be a long shot, but have you tried the second BIOS on the GPU? I believe that most of the R9 280X cards have a very small sliding switch labelled "1" and "2" on the PCB by the Crossfire bridge connector, near the I/O plate. Power off your system and try switching it and power it back up. See if that helps. Sometimes the GPU bios can make all the difference, other times it could be imminent failure and you may need to just replace the card. FYI, if you want to flash the BIOS on your card, be warned, you could brick it, especially if its already having artifacting issues, and it may void any warranty you have as well.