I would suggest trying the full vac port. or checking to see if the advance weights are gummed up/sticky and make sure the vac can is not ruptured and a vac leak when coming back down to curb idle.

mine did this with a bad vac mech advance can. had a slight vac leak made it go lean and stumble at idle.

I run mine on full vac and set A/F ratio with vac gauge with mech vac un hooked.

get my curb idle & initial timing set. hook up to full vac and it idles up and cleans up the idle. then readjust the curb idle back to the 650 rpm setting.

or I use the FBO mech advance plate/MrGasket light springs and set curb idle/initial timming with no vac advance hook up.

14* initial/mech advance plate on 18* for 32* total (no vac advance)

runs about the same way with either set up.