A defective exhaust gas recirculation (EGR) switching valve may cause an intermittent rough or unstable idle engine.Google+
Car Problem Reports
Hyundai XG350 Rough Idle Due to Faulty EGR Switching Valve
Hyundai XG350 Problem
Engine Affected: 3.5L V6
Average Mileage: 131,065 mi (95,000 mi - 180,000 mi)
Visitor, 2003 Hyundai XG350, 3.5L V6, 140,091 mi
EGR valve welded to threads. Finally removed new installed and same problem. Back in shop.