When the vehicle gets warm it has a runability issue while at steady throttle.

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During steady throllle or cruise control the vehicle stumbles and won't take throttle input. If I stop and leave in park it idles fine and runs smoothly through the RPM range. If the key is cycled off then on it will run for good for several minutes and them it will stumble again.The vehicle has 227,000 miles and was very well kept and maintained.