the immobilizer system because it sends a signal to the coil when to fire and also limits the spark (Rev Limiter).
Maybe this will help?
Something like in this post of collbain 2000 from Jan 30, 2007 seems very plausible. (in the link waspmike shared)
The engine starts up and idle is perfect but when you turn the throttle then at a certain rpm the engine sound changes and the engine clearly does not ignite at every compression stroke, it runs irregular and clearly misses several compression strokes in between ignitions.
If engine rpm is indeed limited to 2000, then that is probably too low to have the clutch engaged.
So I would start with checking the key and the immobiliser system of your scooter (or better: have it checked because if there is something wrong with it you probably cannot fix it yourself)