I'm too late to help you, it's now 2019! but hopefully this will help others:

I had the same problem, it was the stator.

My bike always started well when cold, but stalled sometimes when hot. Almost impossible to restart sometimes after more than a 10 minute run. I would have to leave it an hour for the oil to cool down before it started again.

Turns out the stator was shorting when the heat from the oil expanded the wires in the stator, a very common problem apparently. This only happens after the stator has already been fried.
Some say stator get fried by the bike overheating, indicating an oil problem, possible, but it's more likely that the fried stator originates from an electrical problem with the rectifier or battery. If the two phases that charge the battery are not being "absorbed" by the battery, the windings in the stator can heat up and burn through the insulation. This could be from running without a battery or from the rectifier not working and therefore the phases that charge the battery having nowhere to go, thus frying the stator windings.

I spent a long time messing around with spark plugs, carburetor, spark plug cap, anti-parasite, HT leads etc. I was about to replace the coil when the seller of the coil told me about how stators go bad. This could have saved me a lot of time and money if I had known earlier!

Anyway, I hope this helps other people avoid wasting time and money.

Damo.