Hi XXXXXXX;
When it starts to do this look for the carburator linkage to be moving a lot. if it is then grab it and see if you can stabilise the rpms manually. If you can then the govenor mechanism is faulty. This can also be caused by a low oil situation. As the oil warms it is not as efficient at maintaining pressure on the actuator inside the crank case. If the oil is low this compounds the problem.
One other thing thsat can cause this is a weak rich mixture solinoid, If your unit is so equipped it will have a wire going to the carburator. This part will need to be replaced, and the carburator rebuilt.
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Small Engine Technician
Fixed my first lawn mower at 7 yrs old. Still fixin them, and more today