Wow, that was quick! Thanks for the tips!
I do make sure that my feet are normally perpendicular to the ground, but when I really go all-out, I put alot of force into pushing away with my legs, especially uphill. This technique creates long strides, and causes movement within the boot (one leg pushes back far, and the pushing foot ends up being quite un-perpendicular while its braced against the ground). This could be it.
Next time I'll try to change the way I push away, and use some sort of insole. The people I usually skate with always say my technique is really good, especially uphill... but maybe they are just being nice
