The great radiator cap test

I'm fairly clueless about cars. I know enough for basics, but as far as deeper- inner workings go.. I'm clueless. None the less Joe and I have a running joke. For some reason I'm able to accurately diagnose an issue with some clarity. I may not have the words correct, but the problem is. I've been right an oddly high amount of times. Now to my latest venture.My car (prior to the extend break down time) had a problem in high weather. The engine would run hot and when shut off you would hear a boiling sound. Joe never heard it so he attributed it to my imagination. Yeah.. he should learn to stop doing that. Then we had a problem with our Taurus last year (have I mentioned we have bad luck with cars from time to time? lol) and now it also tends to run hot in hot weather and also has the boiling sound. Again it's my imagination as I was the only one hearing it. FINALLY it happened. Joe drove to work and heard the boiling sound. Sigh. Now of course he's trying to fix it. I decide to finally look it up online. Why I hadn't already is beyond me). I posted a bit about this in one of my other car repair posts. I found out that this could happen from a faulty radiator cap. Yep.. a radiator cap. Inexpensive little devils. I tell Joe this and it makes perfect sense to us. The cap keeps pressure on the coolant so that it boils at a higher temperature. I got my car back on Wednesday night. Perfect timing as we're in a heat wave of mod 90's and high humidity. It ran well on the way home, but that was in the evening. Yesterday I went out at the worst part of the day and the temp ran slightly above mid line for the entire trip. I still bought the cap plus one for Joe's car. Joe grudgingly put mine on last night as he was in one of his moods. Then says he's going to put the old cap in the trunk. I caught him dead on when I said "what.. in case I'm wrong?" he answered yes before he thought about it then tried to change the subject. Today was just as miserable as yesterday and my car actually ran a bit cooler. For the most part the temp was slightly under the mid line. Not much proof, but it also didn't run hot while in standing traffic. However it was enough to convince Joe to try it since his car was running hot after getting stuck in traffic. Seriously though - why not try a $7.00 radiator cap. If it doesn't help then it's only $7.00. Tomorrow morning he'll change it before work. Let's see if it makes a difference.