On further thought.
1. It is not your card, otherwise standard benchies like 3D Mark and even BF3 etc would be badly affected as well.
2. For Dx9 games 680 stock isn't that much better than 580 oc. A heavily overclocked 580 will come very close to a stock 680 in some situations in Directx 9 games. Regardless, point 1 is still valid.
http://www.tomshardware.com/reviews/geforce-gtx-680-review-benchmark,3161-9.html
3. It could be some settings in the control panel to some extent, but that isn't the only problem.
4. It is your CPU + MOBO (entire system). Regardless of what anybody says to you, and everybody will. Everybody has a different requirement for smoothness and what I consider stutter most others can't even perceive it as something which exists. So you are perhaps better at perceiving such deviations from perfection than most others with relatively old systems. A OC to 4+ GHz may help a bit, but 2600k at stock will still give better performance in terms of smoothness etc.
5. If you really think it is your card, then fine, go RMA it. If you want, sell it and go for 7970 if you think that would help. It might to some extent as different architectures are dependent on the CPU/rig to different extents and react differently. However, a 670 or 680 will probably not solve anything, even if you RMA, nor will overclocking it since that isn't the bottleneck.
6. You can start with an RMA of different system parts if you want, there is a 5-10% chance that it just might solve it. But 90% chance points to your having to go for a CPU + MOBO + RAM overhaul.
7. Btw, which power supply do you have? Although this may help only in 1% of the cases, and since you had problems in both situations it probably isn't the problem. But still.
Take care.