Ford Crown Vic cooling sys, and steering

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.

this is a monte carlo forum, but i'll help anyways.. as for your cooling problems, check the radiator cap, my next thought would be cooling fans...and last water pump...make sure your hoses arent crimping either especially your bottom one