Honda HR21-5 powered rotavator.

I wonder if that coil IS cutting out when warm?
You can get a cheapy inline spark test for around a fiver. It might show you that the spark degrades just before the engine dies.

another thing to check is the engine cutout switch (and associated wiring) as this shorts the spark out, so if the wire is frayed and touching earth it willcut out.
Grendel