Old fuel? Old plug? Turn the plane straight up, not running, and shake it, do you hear the clunk rattle around, or just hitting the sides of the tank? It should be free to go tap tap tap, but still be hooked on. (hitting the sides, not rolling around).
I've run with a bad glow plug, and the plane would run fine for a few, then sure as s#@t, it would quit after take off. Same can happen with old watery fuel.

Jetts