If the backfire you describe is not so much a loud bang, but just a wierd popping sound, check for a vacuum leak where the intake manifold fits to the head. Try spraying a flammable liquid or propane from a hand-held torch around that area while the engine is cold and idling. If the engine smooths out, you found the vacuum leak. If something ignites the flamable substance you are using to find the vacuum leak - well - you didn't need that facial hair anyway, right?

If you have never replaced the intake manifold gasket, it's due. You'll find that it is rock-hard by now. Intake manifold leaks are worst when the car is cold because they tend to reduce as the car warms up and the metal expands.

At repair jobs go, the intake manifold gasket is cheap, easy, clean, and quick, though there are a couple of nuts that are hard to get a wrench on.