Thank you for all your help.
The stuttering and frameskip is almost solved. I ended up using NVCP exclusively to tweak the values, since I could test it (frame skip and crosstalk) straight away and see if it was smooth, even though NVCP doesn't let me tweak the values much.
Now its almost perfect, its good enough. At first, i've managed to get good results at 239hz, but then i got good results at 240hz also. I haven't tested the limits (vertical frequency) without frameskip. ive slightly tested 242hz but with skips.
So here's what I got:
239hz
and 240hz
I've been tweaking the Vertical total pixels, between 1280 and 1304, for now its ok. The "problem" in NVCP is that no matter what decimal digits i put in Refresh rate (240.xxx), it always resets to .000
I might give CRU another chance though (after removing the custom resolutions in NVCP). The problem i got with CRU, I never knew which modes were actually set. thats ignorant from me, i should have seen some tutorials first. (also the OSD doesn't show actual refresh rate, it always shows either 60, 120, 144 or 240)
I wonder if this kind of overclock does any harm to the monitor in the long run. Also I wanted to know how to reset the timing settings to the monitor's defaults (I assume its the automatic option in the combobox?).
Thanks everyone, and for the ones that i saw complaining about this monitor in this thread - don't. just fiddle around a bit and it will be fine. There are good deals for this monitor if you look for them (<300 eur/usd)