maybe the governor is not working properly ,just a guess.
The simplest thing i can think of is the bleed screw on the injector pump, Ensure this valve is closed. If it's let open, you don't get enough lift pressure to keep the motor running under higher speed or load.
Without seeing it, it's hard to know if the motor stalling out is fuel related or something is loading it down.
I'd like to start with the easier stuff, fuel lines, fuel pressure to the pump. If the motor stalls without black smoke, i'd suspect it's starving for fuel. Black smoke is unburnt fuel, if the fuel isn't there, it can't smoke. If it's still injecting fuel, but not enough air, you get black smoke, same with it being fueling correctly but under high load, it will give black smoke.
It's a spot to start looking i guess.