A radiator may be in your future. I'm not sure how hot it's getting in your neck of the woods (It's 93 degrees F as I'm posting this, in Texas), but typically the hotter it gets, the more problems related to inefficient cooling you'll experience.
Without knowing much about the length of your ownership, driving habits, and several other factors - I would guess that if you're just now seeing this problem, and the temperature is pretty warm, you might be doing well to invest in a radiator. It's your money, so please don't take this as a definitive answer because it's tough to troubleshoot across states, but if you're suspecting the radiator, it would probably be a good place to start.
My logic might be faulty, but it's like this; your radiator is
the heat exchanger of the engine. Hot coolant gets cooled by the incoming air and helps to maintain operating temperature (along with everything else in the cooling system). If it's weak, or subpar, the hotter it is outside - the less effective it becomes. It sounds like you already think the radiator is the culprit, and if everything else seems to be working fine then you've probably found your answer.
Again, it's easy for me to spend your money for you, so just know I've been wrong before. My wife reminds me all the time.
