I began to have problems with my car since I had the dual exhaust installed. Whenever I have the car in park for longer than 2 minutes the RPM drop to 500 for a second then return to 750, it continues to do so for as longer as the car is on. The same thing happens when I come to stop when I am driving. I had the battery, alternator, spark plugs, and the air intake checked and none of these seem to cause the problems. I had auto zone run the test on the computer an no codes came up. I am thinking it might be the oxygen sensor.