aprilia rs 125 reving problem?

Hi there , i have a aprilia rs 125 old shape 2005 , when i am riding my bike goes to 8000 revs fine but then after 9000 there is no more go its like its restricted itself , it did use to do 11000-12000 easy , i havnt checked anything yet , apart from change the spark plug no difference any help thanks

Adam
October 2011