have you checked your spark?, and your fuel line for clogs?... and make sure your runnin a clean filter (a dirty filter on these bikes causes them to act CRAZY)... i know it sounds small time, but those little things can add up to a hell of alot
other than that, it could partially be caused by the wrong coolant mixture... or just plain wrong coolant
and can also be caused by the exhaust valve not functioning properly... but it should still run properly just wont hop off the bottom end as good...
do some investigating, lemme know, ive got the exact same bike
and just a thought... have you done a compression check to make sure everything is functioning normally, and was installed correctly??... cause that could DEFINATELY screw all that up
some dumb ideas but not too far fetched,
LemmeKnow man!, GOODLUCK!