I've got a 2000 Taurus with the Vulcan engine and sporadically when at cruising speeds and RPM's I get a slight RPM surge. It will rise or drop around 100 RPMs several times. The Tach. needle looks like it has a slight bob to it. It doesn't seem to be a major drivability issue, but I would like to fix it. Is it just a vacuum issue and if so where should I start to look? I did notice that it mostly happens if the fan is turned on a non-air condition position. It does not seem to have the surge if the fan is turned off. If not any suggestions would be appreciated. It does not seem to do it at moderate speeds or during idle.