With my 2001 windstar it has an engine shutter while maintaining a constant speed. When the van is gaining speed or going up a hill or coasting the shutter is gone. When I reach a level spot in the road and lets say want to stay at 50 mph with or with out the cruise control engaged the engine will flutter and shack like its not getting fuel or a misfire or something of that nature. The engine never dies at idle it used to but stopped doing that and hasnt for many montths now. Im confused by this because it seems that if i had a vaccuum leak or a upper intake problem that it would do it all the time but it doesnt. The engine idles fine, starts fineand otherwise has no other issues. If you have or have had this problem and or know a fix you would be my hero.