I never said it wasn't related. The thing is, at lower resolutions your GPU usage wont always be maxed. For instance, I can play an older game and have well over 100FPS and not use even 50% GPU. Lower resolutions tend to do this as well because the GPU doesn't have to work as hard, but the CPU does. At higher resolutions the GPU takes longer to process textures and other things and so the CPU doesn't have to work as hard to keep up feeding the GPU information. At lower resolutions the GPU is in like super speed mode because it doesn't have to work as hard, and the CPU has to keep up with that. Hence why I said, lower resolutions are more CPU bound. The analogy is terrible but I hope you get what I'm saying. You can easily Google this if you don't believe me.
Sometimes the CPU can be a slight bottleneck as well, for instance like my i5 750. Even though I am still getting more than acceptable frame rates with the 580 the fact that my GPU usage isn't maxed is an indication that my CPU is a slight bottleneck (1200p + maxed and AA). I'm sure this may confuse you or some, but I'll try to explain. Back in the old days you could tell a CPU was a bottleneck because it was either obvious (way old CPU, way new GPU) or it was at 100% all the time with a good GPU and not giving good FPS compared to other people with better CPUs and a similar card. In this case, it is more likely that the game in question can't and wont max a quad core or higher CPU. So speed and single threaded performance will bottleneck the GPU in pure throughput even though the CPU isn't showing 100%. In your case, with a modern i5 this can't possibly be the case. I figured I would put that there so you don't think it is your CPU. This is kind of why I tell people, if you're doing 60fps just turn your monitoring programs off and play the game. At 60fps average (or around it) your dip wont be enough to lag even if one part of your system may be a potential bottleneck or something isn't being used 100%. Parts not being used 100% to me is just headroom, and we like headroom. The people who get stuck staring at percentages or frame rates usually wind up spending a ton of money for nothing. Kind of giving that warning before it happens.
As to your problems, it's not that I'm getting a temper or anything. It's just as of right now I'm not entirely sure what your problem is and can't help. All I can really say is, it isn't your GPU, it isn't your CPU or any form of bottleneck. It simply lies within your PSU, software, or a bad part.