I had a RailKing SD70ACE with this problem out of the box. Being somewhat, shall we say, "unskilled" in the art of train repair/trouble shooting, I made the bold call to lube the gears a little. I figured it sat on a shelf for at least a year and a half, so it might be a little dry. It was, so a greased the gears a little and started running it. It was still a little jerky, but seemed a little better, so I kept running it at higher speeds for about 15 minutes or so. After a while, it smoothed out across the entire speed range. It now pulls steady at 3 SMPH without any hesitation at all. I did nothing else to it; maybe I just got stupid-lucky, but I thought I'd throw it out here just for fun
-Eric