The car has been to five different tuners or just the PCM? Five different tuners verified timing and fuel, did any point out something to the tune prior to when they looked at it? If not then they all correspond, so look for a mechanical flaw. Check your fuel pressure at idle, ohm your injectors, and if all is good but you're still backfiring, open the throttle blades some with the fast idle screw to compensate for the rich condition (screw is located on the bottom of the throttle body). Is the engine surging at idle?
