3 things that cause that on an rs 125 with the rotax 122 and 123 engine are
the kill switch packing up
the ignition pick up coil next to the magneto packing up.
the catalytic converter coming loose in the exhaust and blocking the gas flow.
i am a bike mechanic and have been doing it for 22 years so im pretty clued up on this model.
adam bike doctor
October 2015