Not sure what to make of it. Seems you've covered the basic parts already. The replacement thermostat could be bad. You can pull it and drop it in boiling water to make sure it opens. I had a similar problem a few times.

One time I did some work on the engine and reinstalled the t-stat upside down (stupid, I know). Seemed to work fine on the road test, but quickly started to overheat when I got stuck in traffic and stayed hot after. That showed up on the temp gauge. I gimped it home real quick with the windows down and the heater blasting on high... in the middle of the summer. <img src="/forums/images/graemlins/scared.gif" alt="" />

I had the ground wire connector on my electric fan fail (poor connection that torched the connector and wire). The fan stopped and the temp gauge read high every time I came to a stop... but it cooled off quick after I had the vehicle moving again. So I gimped it home by avoiding every stop light and "California stopping" at every stop sign on the way back... <img src="/forums/images/graemlins/shiner.gif" alt="" />

The last incident, which happened many years ago, involved coolant puking out of the overflow tank on my Chevy Sprint after driving for a short period of time. I only noticed it after I had stopped and smelled the hot coolant, which had blown all over the engine compartment. It never showed on the temp gauge unless the coolant level got pretty low in the radiator. It turned out to be a blown head gasket that caused a cylinder to pressurize the cooling system via the adjacent coolant passage. It was far more pressure than the radiator cap could handle, thus forcing coolant out of the radiator and the overflow... until there wasn't enough coolant left to blow out. Then it just blew out the residual in the reservoir and started to overheat. I didn't bother fixing that one. Instead, I topped off the coolant, drove it to a gas station next to the local Ford dealer, cooled down the engine and topped off the coolant again. Then I drove it to the dealer and traded it in for a new Ford Ranger. It helped to do the trade just before closing time. The sales people were more interested in going home than inspecting the 3-cylinder "beast"... <img src="/forums/images/graemlins/smile.gif" alt="" />

I don't think the AC fan would have been the culprit. It should only run when the AC is on. The clutch fan should be sufficient to cool the system at idle, as long as it is spinning normally at idle. Is this a problem that only occurs with the AC on?

I can't think of or have experienced anything else that might be suspect. Perhaps someone else has a thought? I'm curious to hear what it turns out to be...

<img src="/forums/images/graemlins/patriot.gif" alt="" />

1997 Sportage 4x4, auto-trans, Warn manual hubs, 4" UPYOURKIA front lift, TJ 106AA rear springs, 2-5/8" body lift, 31x10.50 treads, SmittyBilt SRC front and XRC rear bumper, swing-out tire mount, OBX LSD front diff, Track Finder rear locker, 5.38 R&Ps and... really crappy gas mileage! <img src="/forums/images/graemlins/shiner.gif" alt="" />