Mustang Backfiring at 4000 RPM

Well this is my second post. I was driving my mustang the other day and I wanted to get on it and the car at 4 Grand the car started to backfire like it was missing. It does this once and a while. I hate to mess with these new cars. I pulled a plug and it was a very light color almost white Running lean. I was looking and bank two spark plugs looks like I will have to take the intake manifold off to get them out? I cannot get my socket down there. Could it be the plugs, plug wires or a injector or a fuel rail? I was nervous when it did this. I am also the 5th owner. I will probably end up trading it in for a 07 GT or try to get a bullitt.
Thanks any suggetions please.

I pulled a code p1131 O2s no switch lean. Should I replace my O2S. I cleared it and has not come on again it has been two months since. My problem is that my manifold is covering up part of the spark plug hole and I cannot get the socket down there it won't even go through. If it was your pump was the bad one making noises. Also to throw this is my car smells like sulfer or rotton eggs I was thinking it is the cats? when the car runs lean would it make the cats smell? Should I run a fuel injector cleaner through it? Lucas fuel injector cleaner. I like that stuff because it is a non solevent and says it will not hurt the cats.

Thanks for the help. I get what you mean about filling it up. to see when exactly it starts doing it.