Already tried refresh rate changes but does not help at all. I fixed the stuttering tough, it was caused by some polling done by an app called f.lux, there is a safe mode in it that disables constant polling and stuttering is gone.
Framerate reduction is still present tough, a lot more evident when playing videos. It happens when the stuff is played on the second monitor, the one attached to the onboard vga, and the framerate on the firstmonitor drops. I have read around that this problem should happen when 2 monitors are attached to the same card and not on different cards.
What I also noticed is that if I play videos from MPC using MADVR as renderer it causes framedrops, but if I use system default renderer there is no framedrop, playing videos from browser causes framedrops but a browser causes framedrops in general because it uses GPU not only for videos.
I don't know which older driver versions should I try.
I believe now this is a completely different issue; in any case, I included our latest and previous driver releases. You may as well try the drivers provided by your system manufacturer.