Another thought: timing point at idle.I have been able to help people with a sound engine seeking a smooth regular idle and hearing these irregular misfire/sput/valve leakage type sounds at idle.Many times they have timed the engine by "ear" to get more pep or acceleration at low speeds and have the timing advanced too far.For a stock cam designed for smooth low speed and idle characteristics if spark is advanced more than 4 deg from specified the intermittant misfire is likely.The tricky ones which had you setting the timing with vacuum line unhooked because they had manifold vacuum to the advance chamber will probably jump to over 20deg when the line is hooked up and the questionable idle will be there.
Test my theory.Use a timing light to get an idle setting of 4 deg BTC after verifying that the mark is still very close to where it belongs if it is on the balancer and see if you hear a difference.Maybe even retard it another little bit to see what it sounds like.
If you want smooth regular idle and snappy response too you may find that getting the service of a mechanic with a distributor machine to build a custom advance curve is the way to go.
Compromises abound when we start "tweaking" these engines for our entertainment--but ain't it great?

Bob Taylor