Subaru Repair: Overheat 2002 TS, air filter oil, radiator cap

Question
Great website! I drive a 5-spd 2002 Impreza TS wagon w/ 2.5l engine that has 135K miles.  Reading past posts/answers prompted this question.  About 8 months ago while driving not unlike I normally drive, the car overheated and leaked coolant out the reservoir overflow.  This has not happened since, I live in New England and experience many climates and diverse driving conditions (city and highway).  The (1) time it did overheat I stopped immediately upon the "sweet" smell of coolant, let the car return to normal operating temperature and have had not problems since.  If replacing the radiator cap would help, what pound release should I purchase?  Could this occurrence be the beginning of future problems I can watch out for?  Could my coolant be old and incompetent?

My car also backfires.  When I shift from 1st to 2nd gear is when I hear it most often, maybe because there is least road/engine noise then.  I have replaced the plugs and wires, air filter, oil & filter. Timing belt and clutch were done at 100K. I had it tested w/ diagnostic tester @ Autozone and it read 5-6 trouble spots, one I remember was O2 sensor.  I saved the codes, but do not have them on hand now, sorry.  
Any help you could give is greatly appreciated.
Thanks!  

Answer
In reading your symptoms I have to tell you your Suby is looking for some love.As far as the radiator cap I would only buy one from the dealer or from someone who sells Japanese made parts. But i wouldn't want you to replace it under the premise that it will resolve your overheat problem.As far as the check engine light, the computer is really trying to tell you to fix your car.A real analysts of the car should be done by someone who knows Subaru.The results from just having the codes read may be less than satifactory.Also I say this all the time but it is just plain and simple to important to not continue to point out.

The primary function of the Check Engine Light on a 1996 and newer vehicle is to alert you to a potential failure of a Emissions control device or a emissions control device monitoring system .And while this light is on your Vehicle may be Excessively polluting OUR environment.So Please as you can afford it please get it fixed.

Justin
All Wheel Drive Auto