Could be a faulty spark plug/magneto, which is giving out a weaker and weaker spark as it gets hot. Once it cools again, I presume she starts.
Another option is you've got poor fuel flow to the carburettor, so the float bowl empties & the engine stops. In time, it re-fills & the engine starts again...
When the engine is running, how does it sound? Good? A bit hit-n-miss? Wheezy? Black smoke? Does it seem to labour before it stops, or does it just suddenly cut out? Is it stiff to turn after it stops? How good is the compression when starting from cold, and when you try to re-start after it's stalled?
Other, more skilled people than me, will probably also have a bunch of questions for you. Also - feel free to post a picture or two. We love pictures