Idle “zoom” “zoom” “zoom”...

87 LX Automatic, stock factory carb. After warm up and off choke, the idle starts surging “zoom” “zoom” “zoom” etc, to about 1800 rpm in about 1 second intervals then stabilizes after 15 seconds but resumes when I goose the throttle. The idle is slightly higher than before the problem started. It doesn’t zoom while on choke and only starts after warm up. The problem started suddenly. I did a careful inspection of all the vacuum hoses. The car runs great aside from the zooming at idle. Any ideas?