Try swapping plug leads first, but here's why I think it's a sticking ring. If it were an ignition problem, it wouldn't always fail on one side several minutes into the flight. The failure would be more random. It acts like something is getting hot and causing the problem. Then it fixes itself every time the motor cools down. It's hard to imagine how the ignition would do this, unless it's a metal plug boot loosening from the heat.