How do I fix 12.5 i/c gold Brigs and Stratton lawn tractor?

When the motor warms the throttle oscillates gradually more and more until what ever is controlling the butterfly allows it to go full close and then the motor dies. When motor is cool it idles fine. At full throttle it will run less than one min. Good clean fuel. Not sure what is making the osculating throttle. It is when the motor warms that is cuts out and dies. It is not old junk. I assume it is some tuning.