Need some help guys. 1999 GMC Suburban 2500 6.5L 140K miles has developed a vibration at idle that seems to go away when driving. Almost feels like the engine is imbalanced and it shakes the entire vehicle. Took it my GM dealer and they could not diag. Says vehicle had code P0278 Cylinder 6 Balance System code. Injection pump is less than a year old and injectors have less than 30,000 miles on them. Did compression test and all cylinders were 380+ Swapped injectors to see if made any difference, no change. No smoke or loss of power. Any help as to what could be the issue?