It does not seem to me you are drawing too hard, yet hard enough to keep air circulating through the heater. I sure it's the unit. The one I replaced did the same thing. Got through 1 crucible & then after the 1st or 2nd hit of the 2nd crucible it would overheat. My preferred level is starting with 3 & ending the bowl with a hit or 2 on level 4. I would try it just to see if you still overheat at the lower temps like Labpong & I previously suggested. But remember, with lower temp you need to reduce you draw speed accordingly.