Sounds like its stalling and your clutch is bad, but hey im not mechanic.
But you should give more detail!
Does it idle in neutral and when you pull the clutch in stall?
if the clutch cable is adjusted correctly then your plates are stuck together [been sitting for a while??]...bump it,pull the clutch in and throttle on/off fast whilst hitting the brakes....that should ''brake away'' the plates.....sorted.
There are three possible reason for this:-
First the clutch is not adjusted correctly on the clutch cable.
Second the could be wear on the clutch release piston in the clutch.
Third there is wear on the clutch basket.
Ja.