Have similar problem: 2001 V8, 120K miles. 30 minute commute to work via interstate - everything is fine. 15-20 minutes of driving in stop and go traffic - temperature gauge begins to rise - once all the way to the "H". Pulled over on side of road for few minutes. Turn heat to 85 degrees and eventually head back to office parking lot. Get in car for afternoon commute home - no problems. No visible leaking coolant; mechanic replaced thermostat, says hydraulic fan "ok", sees no crack in coolant reservoir, has bled the system and ckd cap for pressure issues. All OK. So - what gives? Trying to avoid taking car to dealership if at all possible - but will do so if necessary. heard there is a coolant sensor that could be bad - can anyone tell me (show me) where this part is located and how to replace it? Also, where can I get a repair manual for the 2001 LS V8? Thanks for the help.