Muffled While Idling

[bryanmoore@classicnet.net]

This has been a problem for some time now and seems to be getting worse. Lets say I pull in to a store and shut off my engine for a minute or two. When I restart and head on my way and pull up to a stop light my engine will idle ok for a moment but then will muffle down almost to the point of killing my engine. It will then go back to a normal idle or "sound" and I will see a small puff of white smoke. Recently I have started noticing this as I drive at normal speed not just sitting still in drive.

I have cleaned the EGR valve and have recently had my EGR cooler and oil cooler replaced and still no change. It did do this before that work was done, but of coarse it didn't do this muffled problem when the mechanic drove it. Sorry if this is a basic question but my diesel knowledge isn't much.

Thanks

Bryan