95 mustang gt , i was driving it to work today and the trans kept down shifting then shifting back into over drive , it did it once or twice and i didnt think anything of it , then it did it like 4 or 5 times in a row then the check engine light came on then the car died , wouldnt start back up ( had a slow crank ) , had my girlfriend come give me a jump and after a min it started back up , i drove it the rest of the way to work ( like 2 mins ) , any ideas ? alternator or speed sensor ?