Could also be a badly worn cam gear/cam gear retaining plate. This happened to me about 35 years ago. It will cause the cam (at idle only) to wander in and out. Sounds odd, I know, but it will happen and when it does it changes your timing enough to make the engine miss and idle roughly. This disappears at rpm higher than about 7 or 800. The frustrating thing is there is no easy way to fix this and you can check about 10 things before you finally find it. Good luck.
Jon

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end