When I am sitting in traffic the car starts to overheat. I pulled over today, plenty of coolant, and it was boiling. I lifted the reservoir cap for a little to let the steam out, then put it back down, started driving, and she cooled down. What could be the cause?

Last year I replaced the water pump, belt, a few radiator hoses, and don't see any leaks, though did see splatter marks on the side of the battery, but couldn't see where that coolant was coming from.

I have been thinking about replacing the thermostat, but I did have heat. Not sure if it could still be thermostat related or not.

 

I would have it checked as it is probably the thermostat sticking. That would be one guess.

 

+1 for the thermostat ... very easy and cheap part to fix it DIY

you could also check to make sure your cooling system is bleed properly

 

I had an issue of loosing fluid just recently along with an overheat issue. turned out to be the thermostat housing. Its been noted a few times in the other threads but the thermostat housing is thermoplastic that is molded around brass bolt holes. There is a tendency for it to crack, causing loss of fluid and overheating. Also, you may have air trapped in the system which won't self bleed and you would have to bleed out of the two screws. One is near the battery and connects to the heater core and the other is in the front over the alternator.

 

Is the radiator fan running?
The ps fan/ radiator fan on pre2005 model year gen1, without the modificaion, share a single fuse...if the ps fan jams, the fuse SOMETIMES blows, taking out the radiator fan.

 

+1 on the rad fan. Check fuses, let it warm up in the driveway, watch temp gauge, and listen for the fan to come on.

 

My 2007 S was over heating it turned out to be my supercharger PTO which runs the water pump had gone bad. Real expensive problem.