What do you use to stir it with? I know about the flames, in the plastic and since I changed back to my original supplier I have had no problem. The flames you saw, were they next to the cup at the bottom? The reason I ask is when I saw these in mine I stopped. It was making little chips in the bottom of my cup, and with chips on a tempered piece of glass it becomes unstable. Actually it is already unstable and anything will make it crack. The reason I asked what you stir with is if it were say a heavy sorta object like a butter knife that will contribute some to the cracking. When you cut glass you score it first then tap it with an oblect to crack it, same here but the score was the spark. Being tempered it wont just crack it will break in many pieces just like your windows in your car (not your windshield)or a table top. If you let it spark in the micro I think you are just asking for it. And also the sparks make burned plastic chunks that you wont see until you pour. Just my thoughts. Frank