Truck, 1998 F150 Supercab Styleside, with V6 Auto. The engine at idle constantly varies from 750 RPM to 1100 RPM. It idles at the low for five or more seconds, then goes up to the high for five or more seconds. When in gear, it doesn't do it. This engine just blew the passenger side rearmost plug right out of the engine, and it had to be threadserted and a new set of plugs put in. Any ideas of the possible fix for this idle quandry?

 

A vacuum leak can cause that.
That's where I'd start looking.