gilera dna speed problem can ay1 help?

i have a dna 125. had the bike a few months and it was standard. basicaly my bike struggles to pick up speed after 50 mph and cant even get 60mph and i know thats not normal as had a few bikes. so i thought might be a tuning problem. ive had it serviced, it also had new polini variator kit new malossi belt new exhaust and problem still persists its baffaling me now. can any one help please. thanks