Possibly a vapor lock or overheat situation. I had this happen to ours this past weekend, we stopped for a short doggy break and to hit up an ATM. It shut off right before we were ready to roll again. It started back up a couple of miles later. One thing is to make sure you have plenty of oil and consider going to a heavier duty oil. I use 30 wt in mine during the hotter seasons and it seems to help. These things run HOT, I have brown grass spots at home where I had it cranked up to run for a bit and was off the gravel parking pad. When you have temps in the 90's and you are on blacktop, it is going to be brutal for the generator. They are air cooled, and with 140°+ air you are going to get limited cooling.
I too have heard about the added vents from FR but have yet to see a comparison picture.