I have a problem with my Holley 600 (4160)
What could possibly have brought this issue on?
The change might have been brough on by a change in the fuel locally.
An increase in the percentage of alcohol, especially in colder climates leads to a more volatile mixture that wants to boil easier. The fuel suppliers frequently change the mixtures summer versus winter, to my understanding.
I would consider intake manifold gaskets, if not already mentioned. See if you can find the set with the highest insulation properties. There is also 2 tricks for the underside of the intake....
One is to paint the underside of the intake with white paint, to reflect the heat from the oil splash,
Two is to build a "belly pan" for the intake manifold itself, to block the hot oil splash. It is usually a piece of sheet metal that is bent and contoured, to follow the under side of the intake, and he held in place with screws or rivets. It blocks the hot oil, from contacting the actual intake manifold, minimizing the heat transfer.