I'm thinking to widen the firing range a small bit
you might try to tweak the glaze by taking your Cornish stone down to 25-27 and upping the soda spar to13 to 15 to get a smoother melt with out movement, just a starting point for testing
I agree here. You may want to add Ferro 3124 or 3134 frit to widen the melting range in your glaze. Start at 10% and do a line test in increments of 1-2%. Ferro3134 would give the glaze a little more fluidity compared to 3124, if you wanted the glaze to be a little runny.