Just got a used 04 Sebring touring convert a week ago and it has been running perfectly. I took it out on the hwy for a run to see how it handles high speed and it tracked straight and felt solid at 65.

When I got back and pulled into the parking lot it stalled. It did start up right away so I drove it around a bit more and found that my rock steady idle of around 1100 was now fluctuating between 1000 and 500 and running a bit rough.

What could have happened to make this change in idle speed and making it run rougher?

 

An idle of 1100 when cold is fine but it should be idling around 600 in drive when warmed up. It should be slightly higher when the A/C is on but not 1100. It's possible your IAC valve is malfunctioning. It may have some carbon buildup. It can be removed and cleaned with some carburetor cleaner.

John