Engine not cooling properly

1999 Mazda Miata 165000 miles 1.5Liter
(If my question sounds strange, its because I speak girl language so you will have to translate it to yours).

Engine heats up in slow traffic as is generally a problem in Dallas. This has happened before and I replaced radiator and 1 fan (approx. 2 yrs ago).
Now I am noticing the problem even at a higher rate of speed. I was driving yesterday (temp outside 104 degrees) at about 70 mph and noticed air conditioning was not cooling. Checked the engine temperature indicator and noticed it was increasing. I simply turned AC off and on intermittently whenever temp increased. I checked coolant and it seems to be OK. This is not a constant problem and seems to occur only during summer days of extreme temperature. It also happened as I circled parking lot looking for a parking space. Can you give me any idea what might be going on as it gets awfully hot without AC here.