fail safe mode light on 2005 Ford Five Hundred

what causes fail safe mode light to come on every mile or so and completely shut the engine off?

2 answers
usally has to do with fly by wire system which replaces the old cable style. when there is a malfunction, the light comes on and the vehicle returns to dile with a slight increase to at least keep you moving to the side of the road.
you say it shuts off completely?? it should not but there may be a couple of issues here.

As Roy said, electronic throttle body concerns are fairly common for this first attempt at drive-by-wire in a Ford car (Ford Diesel Trucks have been using drive by wire for a while). Throttle position sensors are at issue, and the ETB itself. They had a back order on replacement parts for months back in 2009 on these.