Acceleration hesitation from a stop

This did not happen all the time when it first started, but now it’s starting to happen more often. When you accelerate from a stop it goes for a split second regularly and then it’s like you have no power. It will do that for about 10 seconds and then it goes to normal. It also sounds like a low rumble while it’s doing that. I have taken it in because I thought it was the transmission, but it’s not. The transmission shop tore the whole transmission apart and nothing is wrong with the transmission. I only ask because sometimes in this traffic we have here I don’t have time for it to decide when it’s going to do that. Do you have any idea of where to start to look at this?