I've had an issue that could be described something like you did, but maybe not.
What happens when you open the throttle all the way from standing still?
Does the scoot start/accelerate smoothly or does it engage the clutch violenty?
Mine would lift the front wheel off the ground.. it was a clutch issue, costs nothing
to fix.
When accelerating softly from standing still, the scooter would shudder/shake before
the clutch engaged all the way.
Cleaning the clutch helped a bit.. but nothing helped more then holding both brakes in,
pining the throttle to the max and then letting the scooter move slowly (1mph) while
still holding both brakes and throttle to the max. By the time you smell the clutch burning,
the problem should be fixed.
Never happened again after that, got 14k km on the clock now.