If your running the stock ignition, maybe your mag could be getting hot and breaking down???
How long do you have to wait for the engine to cool down before it will start again???
If its a long time, the next time it quits, spray some contact cleaner on the mag pickup unit and this should cool it down alot quicker.
Then see if it starts right back up or has to cool further, if its your reed valve it will not start until the inside of the block starts to cool.