65 New Yorker with a 413 and a Holley 600 carb. Car would idle up and down. Played with jetting, changed out power valve, block vacuum advance- all without any changes to the surging. Today I had the hood up listening to my surging beauty, Noticed I would hear a click right before it would idle up. Click-vroom. Click-vroom. Traced the sound to the- PCV valve. Pulled the valve, blocked the hose, and the surging stopped! Haven't run into a mystery PCV valve problem since the Plymouth Trouble Shooting contest years ago. Ordered a new valve online tonight. Hope that clears it up for good!