alright i think i found a solution after searching a bit i found on a post on steam fórum of FSX of a guy who had a similar problem, it seems that if you delete the Shaders folder everytime before starting FSX, at \AppData\Local\Microsoft\FSX\ it will fix, im not sure yet but i did the same flight, Brazil to UK 2 times, one without FSXWX and the other with it and i had no stuttering, later ill try even longer flights that are in range of my 777-200ER and i'll post an update