Engine overheating problem

2001 Mitsubishi Gallant 2.4L Auto Transmission
Below are the things that I have checked and everything looked fine:
1 Antifreeze level
2 Belts
3 Both vents are working
4 AC is working
I don't know what else to check. Engine is overheating at slow traffic and works just fine on freeway. Please let me know what else can be causing this problem. Thanks in advance for your help.

 

Air at highway speed can cool the engine without the fans and can also cover up a degraded system.
My first suspicion would be the quality of the radiator
Look at the metal fins for signs of "rot"
I would also suspect internal blockages of the tubes inside the radiator.

 

Thanks for the reply frankiee
If that is the case, then will radiator flash fix the problem?

 

Replace your thermostat while you are at it. It's a cheap fix.

 

A flush certainly would not hurt. I would save the old stuff to take a look at what came out of the system. My first guess would be the condition of the radiator. Aluminum or copper will oxidize between the cooling fins and the cores. I would gently take my fingernails and run them on the fins all over the radiator. Gently not to damage the fins of course. It should have a "tin" sound all over. If you cant gentely rub all the fins over the rad then those fins are likely "rotten" and have no heat transfer capability.
I would still flush the rad and see what crap comes out. That is free and a good idea to do every so ofter anyways.