Recently purchased a 2008 Mazda 3 (4 door shiftronic) with 74,000 miles from a dealership about 2 months ago. Didn't notice it at the time of purchase but after the car has been running for about 20 minutes or so the rpms will start to bounce from 500-800 when at a stop. Does not do this while driving and rarely does it when the car is in park. Brought it back to the dealership multiple times to have them fix it. So far they have done the following:

**Also Note there is no Check Engine Light on**

*Reset ECU
*Replaced Throttle Body (Brand new)
*And just recently replaced the head gasket on the car.

Been reading on multiple forums about this issue. Some say its the MAF and others say it could be the IAC. I talked to my dealership about those and they said the IAC isn't the problem because the Check Engine light would be on. As for the MAF, I don't think its that, talked to a couple car enthusiasts and they say that that most likely isn't the problem but don't know what else it could be. Also I heard the A/C might play into this but this happens regardless if the A/C is on or off. Could use some advice on this one. Dealership is stumped too.

 

In addition to what you have mentioned already could be intake manifold gasket, or a very small vacuum leak