Interesting problem and no help here as I have the exact same thing happening with my 71 350. It happened before I changed it over to HEI and even with:
New plugs
New wires
Timing set at 8+
larger feed wire to the HEI
Carb idle adjusted
etc.
Sounds exactly the same---never on acceleration, only when ideling and not consistant. More obvious if running a little harder then a quick stop to idle---then it feels like one bum cylinder. Haven't put a vacuum gauge on it or checked it at night for spark jump, but would think that whatever is happening, it was happening prior to HEI and after HEI.
Hope someone comes up with a good suggestion on how to find this little gremlin!! Huck
