2004 mercury sable 3.0 gas engine the vin # is u stalling/rough idle, when i drive the car down the road at 55 miles per hour it runs ok just when it is in park it idles bad could be a bad ignition coil pack i checked the Idle Air Control Valve ti is good i changed the mass airflow sensor and the oxygen sensor.

 

I'd start by doing 2 tests.
Get the OBDII codes. Use your own code reader, or have a parts store use theirs.
Check for vacuum leaks by using Starter Fluid. While the engine is idling, spray starter fluid over anything suspicious, the hoses, the manifold junctions, etc.

 

Specifically, what do you mean by "...idles bad..." Does it fluctute from high to low or does it stay steady but seem like its missing etc? You might unplug the IACV and see if it changes the idle. How old are the ignition bits?
Good luck.
Gator