The 3.5 engine of my 2001 QX4 will occasionally run rough for no apparent reason during acceleration. You can feel and hear the engine missing and sputtering. Later, it will be fine with lots of power. There is no consistency to when it happens, but only on acceleration. The Check Engine light has never come on and a Nissan mechanic hooked up the code reading equipment and everything checked out normally. It has 74,000 miles. The engine starts easily and idles fine. I changed the plugs but made no difference. It's a mystery. Any ideas of things to check? Thanks for your help!