I don't know I'm asking you as I've never done it the way you are. I heat then stir or swirl and repeat until I get to proper temperature. If there are bubbles I let rest with something covering the pyrex glass so that the plastic doesn't cool as fast. The bubbles will rise to the top and I take a torch and run it across the plastic to pop bubbles. Sometimes I need to work the plastic again until bubbles are gone. I would imagine if you don't somehow stir the plastic in-between heats there will be pockets in the plastic hotter than others which might lead to browning in some spots.