A backfire occurs when there is an imbalance in the air to fuel ratio required for your vehicle to operate properly. Yes, a lean condition can cause backfires in the intake.
Unless you have verified fuel pressure with a gauge, you're guessing if you have proper fuel delivery. Regulator still in play here. Be sure that your pressure is within factory SPEC. A gauge is the only way to be sure you're not chasing your tail here.
The backfire solution usually involves checking for vacuum leaks, and ensuring the fuel system is functioning properly.
Other things to consider if you have no vacuum leaks and fuel delivery is within spec: Ensure that your ignition is solid. You mention new plugs and wires and that you cleaned the cap and rotor. I would replace the cap and rotor. They aren't designed to be cleaned, and aren't expensive to replace. They can have cracks in them that you can't possibly see. Oxygen sensor can be responsible as well if they fail in a certain way. They have a finite lifespan between 100k and 150k, depending on the efficiency of the engine.
Last edited by tjwalker; 05-09-2011 at 06:40 AM.