03 mustang gt bogs down when shifting.?

I just got the ecu reflashed at the dealer. Car is AUTOMATIC and has 47,000 original miles. Shifts perfectly fine if i shift it manually but when it's in Drive the car shifts fine into first but it bogs down and trips itself really bad to the point where it goes down to 600rpm then when i let off the throttle it goes into the correct gear it should be in. Tranmission fluid was just replaced and has a little over the fill mark which i need to drain but i doubt that is the problem. What is the problem? I would rather get some ideas first before i take it to the dealer or aamco to get checked.