Keeway F-act 50 Enigine cuts out?

My keeyway F-act 50 keeps cutting out when i come up to traffic light or i stop and a stop sign? i loses power when i try to take off and around 15-20mph it starts to die and eventually pick up again about 10 seconds later :S
It takes a few attempts to start the engine if i havn't been on it in a while, ive took the spark plug out and cleaned it and sorted that out. anyone have any ideas?