crown victoria died and mechanic cant figure whats wrong

I was having window problems the other day and to
fix it I shut the car off and when i tried to start it the
car was dead. I got some jumper cables and it
worked for a couple miles, when i accelerated it
bucked and then it completely died. the battery is
new , when AAA came he tested the alternator, it
should be at 13 but the numbers just kept going
down after it got jumped. mechanic has burned
through an alternator and had been there for 3 days
and still doesnt know whats wrong what could be
wrong?!