Gilera stalker with technigas next r = problem?

I have fitted the technigas next r exhaust to my 2008 gilera stalker. At 0-10mph i get a laag which causes me to go slow then at 10mph i will suddenly shoot off. I have taken the variator ring out, de-ristricted the exhaust by removing the restrictions, changed the rollers twice (no change, only in top end & acceleration) and im still having laag... =/ any ideas? Btw do you think this is because i need to upjet? and how much by. Thankyou very much :)