Check these things,
Idle mixture is too rich causing the engine too load up at idle and die.
Idle speed is too low, as your engine heats up your idle speed will drop unless it starts overheating which could raise the speed.
Engine is overheating causing a shutdown. (if the idle mixture is too rich it could mask the problem because the tell tale racing idle won't be there as well as giving you a false sense of having the high speed needle set correctly).
Clutch dragging.
Clutch bearings are going bad.