The rebuild was done by a friend who has extensive knowledge of rebuilding engines. I have asked him about this problem and he advised me to see if there were any riders with similar problems online as his knowledge of 2 strokes was not so good.
The bike runs fine between 8000-9000rpm and was fine beween 8000-10000rpm when the problem initially surfaced a few weeks ago, the "barrier" at 9000rpm has become gradually worse since then.
The oil leak is coming from the oil pump as far as i can see and i've been advised by the previous owner to remove the pump and check it as an o-ring may be in need of replacement as he had the same problem with oil dripping(not spluttering)
Not sure where the filters are, If you could point me in the right direction that'd be great.
As to pictures or a video, i'm not sure how i would do this as the stuttering does not happen so much in neutral or with the clutch pulled in but only when the bike is running in gear, so i'm not sure how to do a video while riding. I'll try and find a way and try and get a video up ASAP.
Also i think the engine may be overheating but as my knowledge of bikes is quite small, i'm not sure.
Thanks, Mike