MY 2003 GRANDPRIX GT LOOSES POWER AFTER WARMING UP FOR 3-5 MIN BUT RUNS GOOD WHEN I FIRST START IT IN THE MORNING

So i have the 3.8L v6 in my 2003 grand Prix gt and i replaced the spark plugs in the car as well as the wires cleaned the coil pack plates checked the sensors and i still cant figure out why the car runs good when i start it up for the first time in the morning but after 3-5 min of warming up the car looses all its power feels like its running on 3 cyl but it runs smooth ( even when it looses power) until i put it in gear then it wants to shut off and die out, sometimes it stalls but for the most part it stays running just really rough like if the firing order is mixed up but its not,even when i drive the car down the street running like this it revs up fine does not shake when I am on the gas, but has no power what so ever i also cut the cat out thinking it was blocked but that didn't solve anything my car has 101k on it. Could it be the muffler blocked or the coil packs, or something else please help me out i drive my 2 year old son to school in it and i need it to run good and be dependable please and thanks