90 Prelude Si - erratic surging idle solution
 

My intake manifold gasket was blown out. This caused massive vacume
leak in the intake causing idle to surge from 1200 - 2500 RPM. Other
weird associated symptom - while coasting above 30 MPH, idle would
remain high at about 3500 - 4000 RPM. When coasting slower than 30
MPH, idlle resumed surging 1200 - 2500 RPM. Replaced the gasket and
the car runs like a champ.