car dies after i put in in reverse

i jus bought a 1998 ford mustang 4.6 liter v8 gt its been running great for
the past week till today i was grabbing a bite to eat  while i was sitting at the
resturant  the car dropped into a very low unusual idol!! i made a strange
face then you know you kinda say hmph lol so i was leaving out the
resturant and made a complete stop and the car dies!! start it back up and
put it back in drive and dies again!! so i panic and the thrown the car in
drive a spashed it and took off, i got the the light the battery hand started to
go up and down so i got worried!! 4 hours later i was trying to go to the
grocery store and every time i put it in reverse and will die so i waited 2 min
i started it back up let it sit, everything is fine, throw it in drive and reverse
everything if ok!!! ??? can somebody tell me whats goin on with this car