thats odd that you guys are experiencing an issue, mine would idle at 500rpm all day long smooth as glass, never miss and never hiccup.

 

I think I will start with replacing the TPS and then the IAC to see if that helps, funny thing is you can hear it from under the hood, but if your sitting in the drivers seat you can feel it, also you can hear it at the exhaust????

 

Possible vacuum leak, or bad fuel being there is only 42k on a 2003 with 10% ethanol in the fuel now-a-days.