Help 02 Ext. Trailblazer

Have a Trailblazer we bought used, in Nov., and had some transmission issues—with 77,000 miles. The dealer worked on transmission and still having a problem in the mornings. When start the car, and press the gas I have no power until I coast about 1/2 block to the first stop sign then it will make a pop under the floorboard where my gas pedal is and something kicks in and I will have full power. Hubby has changed just about everything and been to numerous garages and no one can tell us whtat the problem is. It does not matter if it is warm outside, or cold it does this every morning, and then it rides fine the rest of the day. Any help with ideas of what this is would be greatly appreciated. Thanks in advance!!!