In my experience (operating freight trains for PC/CR for roughly 15 years) this was a phenomenon that plagued the U series units. Compared to the EMD products (35 series and later) the load control of the U-boats was atrocious. Early U25/28 series were the worst, as I recall, in that they would sometimes load tremendously in the lower throttle positions, especially as speed was reduced such as when attempting to glide to a smooth stop when power (stretch) braking. This power surge could be enough to break a knuckle if one wasn't careful.
And yes, when accelerating, every throttle notch would often cause a dramatic drop in load. What ensued next was a slow spool-up of the turbo (and a cloud of black smoke) and redevelopment of power for the notch selected. EMD had developed a system that produced a specific horsepower output for each throttle position. It took years for GE to figure out how to do the same thing.
You might get the impression that I didn't like early GEs. And you would be right in that regard.
We don't know what we don't know.