LA360 Vapor Lock, Heat Soak, OR??

The LA360 in my 70 Duster has recently developed a strange condition this summer after running well with no major issues last summer. The Holley 750 dual feed was rebuilt and a new 2 piece (aluminum outer & phenolic inside) spread to square bore spacer/adapter was installed. It replaced an open plenum similar 2 piece spacer/adapter for better low end. The manifold is a spread bore modified LD340 with Chrysler part # on it, same as last year. 727 trans.
After normal starting & warm-up it is idling & running fine initially. The condition begins around after 30 minutes of driving, it doesn't want to idle when I come to a stop and dies. It will start right back up but I have to keep the go pedal pressed above idle to keep it running. It dies as soon as I let off the gas. IF I stay on it, the motor runs OK at speed, just dies as soon as I let off the gas. Nothing else was changed from last year. The engine temp stays constant around 185. Is the new spacer not as good of a heat isolator as the old one?
Any suggestions or ideas?