2000 328CI Overheats in traffic, Fan not coming on?

[CGrill]

My 328CI keeps over heating in stop and go traffic. The weather has been especially hot lately.
The water & coolant level is fine--I've been checking it to see if its leaking--its not.
It seems like it overheats faster when I have the AC on, though I don't blast the AC.
Could it be the fan that is not going on when it gets hot?
Has anyone had this problem?

How much does it cost to replace the fan?

Any input would help.

[shogun]

If it overheats in stop and go, it is probably a defective visco/fan clutch.
Riugh estimate for a new fan clutch about 100 $ from an aftermarket shop plus installation.