how can I fix my Sukida scooter SK125T-18 2007 model?

It starts OK but after the cold running (choke?) has finished the engine becomes very erratic. Previously, after the same period, the engine would cut out. If I used the starter with no throttle applied it would restart, but as soon as I opened the throttle the RPM would increase and then the engine would cut out again. After several attempts the engine would be fine again, so I put that down to an annoyance. Now I can't predict when the engine is going to cut out.