Your secondary fan is either not working intermittently, or is spinning the other way around. It can happen with a bad capacitor.

 

Every summer I'd have to watch my temp gauge, I noticed my car would eat coolant on hot summer days. My car doesn't leak or any of that such, I actually have a bottle of lexus coolant sitting in my trunk incase it bottoms out. I remembered last year it nearly hit the red mark on the temp gauge, I was like wtf? Pulled over and noticed my coolant tank reservoir was empty and probably half-gallon from the radiator itself. It took about two bottle of Lexus coolant to replace what I've lost. Took it in the dealership the following day and they said couldn't do much being that the car is stable to drive around at that moment. I was like okay? Since then (a year later) my car has been normal, I think my car would only act up if the day itself reaches 110 degrees.

Again this is just my experience and wanted to share this being that we are in the same thread as the OP.

 

I would check the top seal on the top plastic of the radiator, the front core support covers the top of the radiator so its hard to notice. Maybe if you look under the core/radiator support you might find some dried pink foam. If you see any dried pink foam thats toyota coolant leaking. This actually happened to me

 

If your car overheat after driving for about an 1hr like you mention thats mean the radiator cannot keep up. It is cheap to replace the radiator. You can tell that the temp is closer to overheat when you are driving on the freeway even with all the fans are on. On cooler days, it will not overheat.

 

Same thing happend to me. Re filled the coolant through the reservoir and car running. Until the other day, I re filled, and it kept overheating. I took it in, and the shop says I need a new coolant fan motor ($250 from Lexus). Going to have him replace it, along with the radiator cap. (about $400 total)

 

Have you inspected the radiator fans for yourself to make sure they both are bad? I say this because its pretty rare to see a 3GS radiator fan motor go bad. Ide hate to see you change it only to keep chasing a fix by slapping random parts at it. Did you ask if he or she mechanic 12v probe that bad fan motor?

 

I did not ask for that. This guy is the same mechanic I've been going to for some time, so I do trust him. He couldn't find a used one at any of the nearby junk yards, so we had to go with Lexus.

 

Theres two kinds of mechanics, the ones that take time to properly diagnose and issue and then there are the parts slapper. A part slapper mechanic will scan something briefly and come to a quick conclusion and will start ordering parts in hopes of fixing problems. The mechanic that properly diagnosis an issue will scan and will test those parts that are bad, before ordering anything.

Next time you can use www.car-part.com to search for used parts in your area. Or ebay is also good!

Its just rare to see radiator fans on a 3GS go bad, your the first that I have seen.

 

Clean out the radiator with water it's dirty