2001 Chevy Impala Engine Surges when accelerating slightly

My Impala has 128,000 miles, but it has always been a very reliable car and we have been meticulous with the maintenance on it. Recently, there are small surges of power in the engine when you are slightly accelerating on any type of grade. Our mechanic, checked and there are no fault codes. He also did some research and thought that it might be some knob that controls the engine that has been known to get flat spots over time. He replaced that knob, but the engine surge is still there. At 100,000, we did a full tune-up on it with new spark plugs, fuel injector cleaning, etc. Any ideas?

Thanks!