it sounds like it may be electronically limiting the revs,is it a chipped igintion model i.e does it have a red flashing light on the speedo somehere,if so its common for the immobiliser to lose its record of the keys chip and may need re programming using the red and black key,if you dont have that then try a new unchipped box from electrex they have a website
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix motorcycles.