Rough idle... only when warm.

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I had a similar issue with an engine a customer brought into the shop one day- very different as it was a 440 with aluminum heads, aluminum intake... the engine builder only used a stamped valley pan... what happened was the expansion of the heads/intake were so much that a vacuum leak developed on the intake track of cylinders 2/4 and leaned the cylinders out to the point they wouldn't fire... caused the engine to idle fine when cold but started running rough once it warmed up.
Again - very different engine but that's something I have personally seen create the symptom you listed.
 
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