I would make the model just regular polys and delete the split poly lines you made, while still in poly mode, edit delete buy type history and remake the lines using split poly tool again. Pull out the shape of the mouth in poly mode, do the majority of your pulling and pushing in regular poly mode and when you want to check and modify the shapes with a bit more control than covert into sub-d to pull vertices.

I found working in poly proxy mode is a pain and for some reason causes a lot of glitches like you said slows your computer down and some times the vertices act up.

Hope that helps!!! It has happen to me before and thats how I fixed it.

Kurt