slider does not work well in certain conditions (more images or more large)

I try to explain this strange problem:

(I use the field version with nodeblock, slide.)
Up to 10 images of 960 px, or up 12 images of 800px , good.
But if I have 11 or more images, the problems begin:
If I slide from the image No. 11 (and following), any target (eg n.10) will be reached, but through the wrong direction: right (forward) instead of the left (backward) and animation begin often from first image.

I made many tests and tried to figure it out for days studying the logic : (but I'm not an expert and therefore I have to give up!)

I have discovered that the problem occurs (resizing the browser window and inspecting via firebug the property 'marginLeft' of ul.slides, which changes dynamically) in all cases in which the animation start from an image that has caused marginLeft of ul equal to or greater of (-)10000.

And this can happen with these combinations, for example (case no loop):
11 (and more) images of 960 px (marginLeft = -1* 11 * 960 px = -10560)
or 10 (or more) images of 1024 px.
or ...

Now they have added transitions css3, (js flexslider v1.8) gave me this strange problem only in no-webkit browser as firefox. I tried also with simple bartik theme.

So.
I suspect that the problem is in the library and not in the module, -I had the same problem (not solved) even in slideshow integrated into the theme AtCommerce, which uses the same library- but in the library demo html everything works perfectly even in these cases (i played around), and therefore would not know what to ask them,
I decided to talk about it here, maybe someone more experienced can figure it out, or put me in the right direction!

Thank you in advance, and sorry if it is not the right place to ask.