While in gear while brake is applied, my 08' Tribute with 96,000 miles is running rough and RPM's drop. Does not die or stall, just runs rough causing a steering wheel vibration. But while driving it is smooth with no problems, and when I turn off the AC it is smooth with no problems even while sitting at light. I've changed the plugs, boots, air filter and fuel filter so far with no noticeable change at all. Any suggestions?

 

Check ignition coil/coils, HT leads ( if applicable), & for vacuum leaks.