have you guys checked your egr valves? when going on the high way, this valve closes for so that the engine runs better. well if there is carbon build up on it - it would get stuck and remain close, so by the time you let off the throttle, it would starve the engine of air, causing the engine to stall. this valve opens only when the engine is accelerating under LIGHT to MODERATE load or during PART THROTTLE. It is closed when engine is at idle, cold, or WIDE OPEN THROTTLE.