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3rd Gen Ram Tech2002-2008 Rams: This section is for TECHNICAL discussions only, that involve the 2002 through 2008 Rams Rams. For any non-tech discussions, please direct your attention to the "General discussion/NON-tech" sub sections.
Are you the same guy who hadn't put the sensor back into his home-made CAI yet? If you are, and still haven't done that, I'd suggest starting with that.
If that's not the issue, then as the others said, check the coolant and the fan might need to be replaced.
The clutch fan is RPM dependant, so when you are idling, it is not pulling as much air as when the engine is running at cruising speed. An Efan cures that problem. A coolant flush is not a bad idea either, because if you are overheating quickly while at idle, your coolant probably is not circulating through the radiator fast enough.
Rad low? was there any coolant in the overflow? If there was coolant in the over flow and your radiator was low enough to cause you to run hot at times then your radiator cap is prolly bad also. They don't cost much and they should be replaced every now and again.