This sounds obvious, but with the ac condensor, and fans up front it's impossible to make sure the cooling fins are clear of debris. I wound up taking the radiator out of my car and hit it with a hard stream from the hose. You would not believe the amount of crap that was in there. The car is much cooler at idle now.

You might try to just blast it out from the front using the high pressure spray wand at a DIY carwash.