I have a problem with my Holley 600 (4160)

when you were in the carb, did you raise your float level? you asked what may have brought this on, I believe this is what happened. If the float level is too high it allows the hot fuel to rise up to the idle circuit and run down into your intake.

If its vapor lock, you should be able to take the air cleaner off after a long drive and after a few minutes or so, you'll here the fuel hitting the hot intake and "sizzle" The spacer will probably still fix it. Just a guess as to why this just started.
Dietz