'02 XLT Overheats after exit fwy and idle

[workinonit]

4.0L 2WD After driving for about 15 mi above 60 mph, if I slow to 20 mph or get in stop and go traffic, overheats quickly. Will idle for 1 hr without overheating. Performed coolent flush. Changed thermal fan clutch. Changed thermostat. Removed water pump and checked impellers (looked good and turned freely). Changed thermostat again (ensuring no air bubble below thermostat, thought I might have damaged first one). Cools quickly if I stop for 5 minutes, restart and turn on heater. Cools if I drive at fwy speeds. Thought it might be sensor but it does get hot. Infrared thermometer shows even temp across radiator. Performed pressure test, no leaks. I'm thinking about changing water pump anyway ($100) or even the radiator ($$$). Can you think of any other tests to perform? Help! This is getting frustrating and expensive.

 

[pax]

You can check the coolant temp sender resistance. On my 95 when it is at normal operating temperture 195 degree, the sender resistance is 40 ohms with the gauge lead off the sender.
If you see 40 ohms and your gauge was at Hi temp. It is your gauge or the gauge wire is shorting out.