2007 Chevy Impala

Engine Performance problem
2007 Chevy Impala Automatic 91000 miles

My Impala sporatically will hesitate to accellerate. This occurs randomly whether I am just starting out or the car has been driven for awhile. When I apply the accellerator, it hesitates for about 20 seconds and then will catch and continue. This problem could occur every week or not for a couple of weeks. It usually only happens once and then is okay for the rest of the trip. The car dealership didn't think it was the transmission and since we can't duplicate it because it is so random, they said to bring it back when it is hesitating more frequently. Any ideas what could be wrong? I don't like the idea to wait, especially if it happens when I am pulling onto a road.