This is a pretty common problem among horses who spent their entire lives on flat, grassy areas like many horses do. If they spend all their lives in a flat paddock or in a stall and are never ridden out of an arena or a track, they begin to think that the entire world is flat and smooth. Then when they end up in an area with rocks or holes or even mild hills, they don't know how to adjust and that results in stumbling.
Ground poles will help, but IMHO, there is nothing better for these types of horses than to ride them at a walk through slightly rough country. Nothing dangerous, but plenty of cattle trails and a few washout places. Hills, trees, deadfalls, etc. Working them in an area where they really have to think about where they are putting their feet and how they are using them will help a bunch.