So another thought is the starter bushing wire has started to come undone. It might even be touching the starter cover (which *might* short out the electrical system and cause the kart to stop running on the track). If you only have a thread or two of the braided wiring in the bushing connected I could see that causing the starter issue. And the bushing wiring is a known item that wears over time. If anyone at the track has a spare starter you could try swapping out and seeing what that does.
If you race Rotax, a spare starter is a must for your spares kit.