I'm not by any means an expert on this car, but I did have to fiddle with one I bought at York a few years ago to get it working properly.
If it's new old stock and has been stagnant in the box for 20+ years (as I am pretty sure mine was), I wonder if the film strip is maybe a bit "formed" to the point where the locations that were wrapped around the sprockets at the ends may have a somewhat permanent curve to them from taking the shape of the sprocket. That's not unusual for aquarium cars, I don't think.
That might cause the motion to speed up and slow down as the film tries to rotate. Carefully removing it to check for this may help figure it out.
-Dave