I currently have the same issue on a 2000 E-350 (V10) passenger van. I also owned a 1994 F-150 (i6) super cab that had this problem. I took the F-150 to an Auto A/C shop, and ~$350 later, supposedly going through all the vacuum lines and swearing there were no vacuum leaks, it still wasn't fixed. I would think that this is a more common problem than a search here would indicate. Anyone else with other/more input?