Engine starts/idles fine but in Drive it stumbles/looses power then runs fine.

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The engine starts and idles fine and initialy runs fine in Drive, but then it stumbles/looses power for 10 to 15 seconds then it clears up and runs fine again. A few times the engine has died after it stumbles/looses power, but it has always started and then runs fine. It does this same secquence each time you shut it off and let it sit for 30 to 45 minutes then restart and begin driving again. Initially when this problem started the "Check Engine" light came on and stayed on for a few days, but it has since gone off and stayed off. Any ideas what is causing the engine to stumble and loose power?