Overheating on warm days (all summer)

What is your question?

If the outside temperature hits 85+, it overheats, if forced to run under 35-40 mph. Idling in traffic causes it to get up near 250 degrees, and when I stop, the overflow reservoir is boiling over. On cooler days it runs 185-210 degrees. On warm days, it runs at 210-235 degrees, even at 35-45 mph. At highway speeds it runs near 200 degrees, on hot days, but quickly goes back up above 235, if forced to slow or idle. I've improved on stock fan, tried "Water Wetter", haven't gone to a Flow-kooler water pump, yet but am perplexed about how the car's operating temperature can be so dramatically affected by the ambient temperature. I've never had a car be so affected by warm weather. I've talked to a few people who said this model has a history of running hot. Is this true? Is there an effective fix, out there? Thank you