360 won't idle smooth...can't figure why

I believe that the clue here is the cylinders involved and the fact that the intake is a dual plane, I had a similar problem with a 318 and a factory 4bbl intake where the casting between the paired ports had some flaws and the gasket wasn't sealing properly. This caused the paired cylinders to be lean at idle. There was no missing under load or at cruise and the plugs looked normal. I replaced the intake and no more problem.