This problem appreared around 100,000 miles. Car would idle roughly as RPMs dropped well below normal when car was in gear. Usually worse with A/C on. Replaced too many parts to list, but here's a few: Both O2 sensors, Idle Air Control Motor, Spark Plugs, Wires, etc... The final fix turned out to be the fuel pump assembly. I had to purchase this from Mazda as the aftermarket ones are not the entire assembly and are visibly inferior. Once this went in, it ran great again. Milage is now in excess of 166,000.