Piaggio NRG Extreme starts but dies when trying to ride?

I have a NRG extreme 50cc 2000reg(X). Just recently replaced all engine bearings, carb, generator/stator, cdi unit/ht lead, polini variator kit, polini clutch kit, auto choke. Cylinder and piston/rings replaced a year ago. Can get the scooter started and ticks over lovely, revs nicely, but as soon as I get on it and try to pull away the clutch engages moves about a foot and then the engine loses power/revs and stops moving, the engine will then pick up revs and sit there ticking over, if I release the throttle and open a small amount the engine picks up and ticks over fine. Open the throttle to 3-5000revs alls fine, but open it to pull away the engine just seems to choke itself.