The A/C in my car gets hot when in traffic or parked?

My car is hellish when I'm stopped at lights in the afternoon; the inside air sometimes feels hotter than the outside, and the vents blow warm air. I was thinking it could be low refridgerant, but it gets really cold when driving faster than 30 mph. It also stays relatively cool even when parked/idling IF it is the morning or at night, when it isn't very hot out. I live in FL though, so usually the temp outside is around 80-90. One thing I've noticed is that the fan in front of the A/C condensor (I think that's right?) never comes on. Shouldn't it be on all the time, if I'm using A/C? How much would this cost to replace? It's a 1998 Honda Civic.