I think you've answered your own question. If you want a steady reliable jumper don't waste your time - or the sellers! - seeing one which is too fast.
As has been said, you've answered your own question. Sometimes horses can be slowed down, sometimes it's their own lack of confidence & balance that's causing them to go like this. But sometimes it isn't & it's just their way. There are plenty of horses out there, buy one that does what you want rather than one that you may be able to change into what you want as there are no guarantees that you will be able to.
