I'm having trouble with my engine. Every time I suddenly slow down coming from a higher velocity, the engine goes down to a very low rpm (only 400 or lower) before going to the normal idle speed but sometimes it even stalls. I tried adjusting the idle speed to raise the rpm but it eventually returns to 600-700 idling speed and didn't solve the problem. Could it be that the timing chains are worn out already? PCV or EGR valve problems? Valve adjustments? I have recently changed the engine oil. What could be the possible solution for this? Any insights will be very much appreciated.