You know, this summer we had several weeks where the temps got to or above 100F(38C) almost everyday. We even had a few 107F(41-42C) in row. I didn't see much of a drop in production, though. I suspect the heavy layer of mulch I put down, and the general weediness of the garden at that time kept the soil temperatures from getting excessive.
So, this would be my long-winded way of saying that bent is probably correct when he says the temperature of the roots may be the issue. Consider wrapping the pot in foil to reflect some of the heat away since it looks like you have it mulched pretty well on top.
It is a healthy, stocky looking plant. It may also just need a little time for the excess nitrogen to be flushed out of the soil. I like to tell people that patience may be a virtue, but it's not one of mine; however, sometimes in gardening, it's the best cure.