400E '93, 68 K. For a few months, I notice knocking when accelerating. It is intermittent, seems worse when A/C on. It won't do it if starting slowly on a level ground but it almost always does it when starting or accelerating in an incline. The engine vibrates, and it is slow getting up in speed. Once up to speed, it goes on smooth. On occasions, the idle will be rough while at a traffic light. Weather / temperature does not matter. It will do it after a night rest or if I 've been driving for a while. Went through a 60 K service a few months ago, including fuel filter, Plat + 4 plugs, Ox sensor. Oil changed q' 3K. I have used additives in the gas, it does not make a difference. Used Shell premium until recently, my mechanic recommended changing to Chevvron, no difference. What is the diagnosis?