Weird fps fluctuations

I have 900/1175 AMD 6850 with 12.4beta drivers, the same issue occurred with the 12.1 drivers, and an Intel E7200.
Fps typically ranges from 45-60 being quite unstable, but every ~10 seconds or so, it gets as high as 111, even with vsync enabled.
For settings I use everything maxed, except shadows are off. This fps fluctuation did not change when I set everything to low.
I must add that it was on 1920x1080 fullscreen, but not on 1450*1050 on windowed.
This happened in Riven Crossing and Forest Encampment, I'm not sure whether it's a global problem.