Crown Vic coolent system.

I have a '98 Ford Crown Vic. I recently moved to Tucson Az. During the summer months when the temp goes above 100 deg., in stop and go traffic, the coolent heats up such that I need to turn off the AC in order for the temperature to drop. If I don't The AC drops out itself. I changed the thermostat, and checked to see that the fan turns on when I turn the AC on. I had the coolent system flushed in Oct. of 2003. Any sugestions as to what the problem is?
Also, about a month ago I noticed that the steering started to get sluggish. The tire pressure is 30 psi ( What is the recommended pressure for a crown vic?) I checed the PS pump resiviour. Again, there seems to be a temperture related problem. I noticed that at night when it cools down the steering improves. Any comments, as to what I should look at. Thank you, Argentfox.

iwould try putting in a lower temp thermostat and as for air pressure it chould be between 32 and 35 depending on prefernce

Check the cooling fans to be sure they work on both speeds. This is a 2 speed fan system. Tire pressures are dependant upon the style and size tire you have. Each of my techs has a different tire pressure for the vehicles they work on. Generally the maximum pressure is for a fully loaded vehicle. Most times there is not that much weight in car. So anywhere from 28 to 35 will be OK. 28 for a softer ride, 35 for a more firm feel.