temperature problems...
 

I have a stock 1990 gxl...

I just made a 100+ mile trip to the beach yesterday and today. It ran at the right temperature until I got stuck in traffic. Not even having the heat on high could control it.

I've read some threads and it sounds like it may be my fan thats not working properly.

Also, when i have the air conditioning on the temperature heats up very quickly.

What can I do to fix the problem? Would buying a larger radiator help keep the engine cooler?

On a better note. Within 3 days my mpg has gone from 15(mostly highway) to 24(highway), and I haven't done anything to my car.

Sudden overheating is a sign or engine coolant seal failure or a bad/stuck thermostat.