After warming up a few minutes with smooth idling, the idle then
starts galloping with a cycle of about 3 seconds and an RPM change
guessed to be from 700 to 1500 or there abouts. On cooling the whole
cycle can be repeated i.e. smooth idle while cold, gallop when warm.
When driving (216k) it runs fine with good mileage (22-24 at 55).  No
apparent other issues: wires OK, coolant OK, plugs OK.  Where should I
start looking?