Idling problem with F350 pickup
 

I have a 1996 ford F350 with a 7.3 diesel 260,000 miles runs great down the road , good fuel millage , good power, but when i come to a stop or slow down the idle drops to about 550 and the engine vibrates like there is a miss, but if you manually raise the idle to 700 on the tack it smooths out ???? . compression is great 400 + injectors cheek out ok, did replace wires and the injector on cyl. #4. but still does the same shaking when comming to a stop . ??? any ideas?