My former work van (a non-Ford) would do the same thing after the battery was unhooked...it would start up fine but I had to keep feathering the gas for a few minutes before it would stay running at idle. Never did figure it out because the battery wasn't disconnected very often.
'03 Mercury Marauder (The Blackbird)
Trilogy Motorsports supercharged #61
Too many mods to list / Driveway Queen
'04 Grand Marquis LS Limited Edition (Spruce Green)
Daily Driver (Thanks, Adam!)
'00 Grand Marquis GS (Silver)
Rotted frame and broken transmission crossmember