Okay, .... No problem on this end. Just didn't want to give you the wrong diagnostics.
As far as "hard" failures go, the computer is the main brain of the whole system. It controls the relays that control the fans. I think there are 3 relays and the computer.
The big factor here is wiring. Open wires, shorted, or grounded. I don't know what kind of experience you have, but finding wiring issues can be tricky. You could start by just replacing the 3 cooling fan relays, and if that doesn't fix it, then you know you either have a wiring issue or a bad computer. We (I) have seen both in the field.
There is a coolant sensor in the lower intake, but the reading goes to the computer and the computer turns the fans on based on the reading.
I do realize you said that when you drive down the road things are okay. And I know you understand that the air flow across the radiator dissipates the heat and you're okay. However, if you do have any cooling issues, it is magnified at an idle. If the radiator is partially plugged with sand or debris, it is going to show up more prominently at an idle.
But I do read where you say when the fans come on, it's okay. I only mention this to make sure we cover all the bases. Water boils at 212 and you gain 2 degrees per pound on the radiator cap. Add 16 degrees and now water is boiling at 238 or 240 range. Now make it coolant, and you raised that up to about 275 or 280 depending on your concentration.
Your cooling fans should come on anywheres between 224 - 228. And then shut off around 200 or so.
I have included a "flow-type" chart diagnostics for your cooling fans. It has the schematic, some description. The file is in a WORD format. CLICK HERE
to view the file. Let me know if you have trouble viewing.
The diagnostics do call for some special tools to diagnose. I thought you could read the chart, and maybe it will give you some ideas.