First, replace glow plug regardless if you think it is ok.
Then, make sure the crank shaft is not slightly bent.
Also, pull the hi needle screw and make sure it is not bent or damaged.
Also, check to make sure there are no air bubbles passing through your fuel line going to the carb as you get to the point to where the engine begins to cut out. This will definitely cause what you are describing.
Is this the OS 75 AX engine?
Sounds as though this is a simple glow plug issue.
Worse case senario would be bent crank shaft or bent piston rod from the sudden stop of the engine.
Keep us updated.

Jim