Question: Your response to my question about vixl was very thorough, thank you.  However, I have some more questions.  Could the vixl be caused by the deep, loose footing of the indoor arena?  I ask this because until the beginning of November I rode outside on a hard packed surface.  My horse started having this problem in late November.  My horse used to feel like he was bucking on the short ends of the arena, but he was actually switching his back feet.  He hasn't done this since he started looking lame.
My horse was a pacer and is very pacey under saddle.

As the horse is so pacy, and responds in such a way to the arena, it obviously doesn't have the balance to work properly in the arena.  If it can work in a ring that is shaped similarly to the arena, but simply with a different footing, then it's quite likely the reason.  In a deeper footing the movements of the horse change, and it can maybe be more likely to start having problems with where to put it's feet.
Deep footing is, in my opinion, quite difficult in most cases for Icelandics to tolt/rack in, and my guess is that it's the same with other gaited breeds.  The horses get pacier, trottier, start to roll, or simply can't rack at all anymore.  It might be sound for the horses to work there, but I'd never work in such footing except in the basic gaits, walk, trot and canter.

Lukka