cutting out ?

hi i have a yamaha aerox with a polini 9 speed variator, mallossi clutch, poini torque spring, mallossi air filter, stage six reed valve and its been upjetted. recently when ive been pulling up to a junction the bike begins to bog and will cut out if i dont keep the revs up. im gunna change spark plug cap and plug as the cap on there is pretty shitty. has anyone else got any other ideas what it could be ?? i was thinking maybe the cdi could be causing it but imnot sure ? Thanks.