First try new spark plug. If same problem do a rebuild of the carb and be sure that all screens are clean. Most likely this will not be the problem… but good practice anyway.
Next check to ensure that your fuel tank is venting well. If it is not, then you may be creating a vacuum lock type of situation that just happens about the same time engine is getting up to temp. To check, as soon as it stalls unscrew fuel tank cap slowly and see if it releases any pressure. You want no change.
If, those are all good, you most likely have a bad TCI. We have had a couple time where were chasing what we thought were fuel issues but ended up being a bad TCI! When they are going bad they start to show it when they are hot.
If none of the above solve your problem, you may be having an issue with some part of the motor losing its seal when getting hot. This could be caused by improper torque of cases or jug or head. All or any of which could lead to an air leak and a lean condition.
My gut says bad plug or TCI… especially if the slowing motor also made popping sounds.
Hope that this helps. Tried to take you in a progression of $$ you have to spend to test/diagnose.
