Really need to get the OBD code. Try a different scanner at another shop. And you can also do this to maybe figure out if it's engine-related or transmission. Next time it happens; back off the gas completely, put it into neutral, and then rev up the engine. You don't have to go overboard on reving it up - just punch down and let up a few times. The engine will either be rough (misfiring), or it will rev and sound fine. After that, with the revs down, put it back into drive (I'm assuming it's an automatic) and confirm that it's still not accelerating properly.