Going off of what your describing I ran into the same issue on my truck last year went through did a compression test and found out I had a bad head gasket. It would chuff white smoke while idling and had a slight miss fire(bearly noticable) and when I pulled the heads off I found one of my cyl. Heads had a small crack. First thing I would do personally is a compression/leak down test and see what you have. I can't remember what compression there supposed to have off hand but maybe some1 else will chime in that does. hope it helps.