I noticed it as the train was slowing down, entering the station.
It definitely wouldn't hurt to make sure the gearboxes are properly greased. However these are Odyssey, not Legacy. I couldn't find the parts diagram on Lionel's website, but I'm guessing they lack the back-driveable gears which were later adopted on many Legacy diesels. It could be that below a certain speed, one motor no longer wants to turn, and the other motor is straining to overcome the drag of the locked gears. The result is an "inchworm" or surging effect seen in the video. What happens if you run only the powered loco with no cars? My hypothesis is that it would be more noticeable.
There is a feedback loop in which Odyssey determines the loco speed, and then commands the motor with a given voltage to maintain the desired speed based on load. It looks like the whole system isn't reacting fast enough. Specifically, it lets the train slow down too far, and then overcompensates causing a surge. My $.02.
