I have a 96 pont grand am that will not start all the time.

I have a 96 pont grand am. I have not had any problems until two weeks ago. I drove it
about 2 or 3  miles, parked it and turned it off. Ten minuts later I tried to start it and it
would not start. The lights and radio still worked.  When I turned the key it sounded like
the car would start, something like when you are just running out of gas, which I wasn't.
About six hours later my husband tried to jump the car, it still would not start.  The car still
sounded like it wanted to start but would not catch. We towed to a mechanic.  The
machanic kept it for about a week. He drove the car and started it numerous time ach day
and could not get the car to duplicate the problem. I picked the car up and drove it for a
couple of miles, shut it off, within two hours I tried to start it and I had the same problems,
it would not start. Next day it started, ran a week and then this morning it would not start.  
Now when you turn the key it sounds like someone trying to start a car that is already
running. The car has about 160,000 miles on it.  Any help is greatly appriciated.