I have the same problem. Ran perfect for the longest time but then toward the end of fall, I was getting the same performance issues. This was on Debian Jessie. December I completely upgraded my system (motherboard, SSD, RAM, CPU, GPU, fans) The only thing I didn't touch was my PSU or the case. I also switched to Arch Linux. Thinking I wouldn't have the same problem, that was the first thing I tested after getting my system all set up. Same thing. I don't really see any reason for it. And there aren't many people with this problem.
I thought it might be an Nvidia problem, since I had an Nvidia card on Debian (GTX 770) and on Arch (GTX 980) but if you're having this problem with ATI, that's obviously not the case.