Sometimes the car stops a 4th gear, even though it is in D.

What is your question?

Sometimes the car does not feel right. While driving,(@ 35mph) and the gear is set in D, it downshifts directly to 3rd gear. I will stop, and restart the car, and the problem goes away. It downshifts from D to 6th gear. Is this a transmission problem, or a black box problem? The problem began 2 days ago after 3/4 days of 100+ degrees.