HVAC Pros please- Window AC unit trips breaker when it gets in the 90s outside?

This is a brand new 15 amp unit (less than a month old), - the dedicated breaker is 15 amp- it runs fine until it gets 90+ degrees outside, then trips the breaker about every 20 minutes until it cools off outside, ventilation isn't the issue, we live on the 3rd floor and there is nothing obstructing airflow. The old unit we replaced had the same problem- I've googled for an answer and am not having much luck- everyone suggests replacing the breaker, but it seems this never solves the problem for the person who posted the question. Is it possible this has to do with the power company and peak load etc? It really sucks that the ac goes out when I need it most.