Could be any of a couple different problems. First and most likely is a fuel restriction of some sort--either at the tank (not vented, debris plugging the outlet) or in the carb (float mis-adjusted or fill valve not opening) or carb jets plugged (dirt from carb bowl) or not adjusted properly. The other possible or less likely could be a bad condenser or coil, loose plug wire, worn or fouled plug, loose ignition wire vibrating.
Try this, next time it starts to stall, pull the choke out some. If that helps, then you've got a fuel supply issue.
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