1995 Grand Am 4 door Sedan won't start.

I have a 1995 Pontiac Grand Am 4
door Sedan that has over 200,000
miles on it.  I just got the car in
November of last year from a
elderly couple so I know it has
been taken care of with regular
maintenance.  I'm puzzled
because the car won't start and I
drove it the day before with no
problems.  I have been driving the
car and a couple of times it
started running rough like jerking
real bad and cut off after going
down a hill and starting back up
right in the middle of the road.
This only happened maybe 4 or 5
times since I got the car in
November.  The last day I drove it,
it ran fine. But the next morning I
went to get in it and it wouldn't
start.  I had a dead cell in my
battery so we changed the battery
for a new one. No service engine
light ever came on. But the new
battery didn't fix the problem.  
Three mechanics have looked at it
and no one can find the problem.  
The spark plugs have been
replaced,  the computer has been
checked and it is good, crank
sensor is good and the fuel pump
is good. All of the fuses have
been checked and everything is
good with them.  Can anyone
offer any suggestions as to what
could be wrong?  Mechanics have
told me they have checked
everything so racking my brain
here.  I don't have anymore
money to keep paying mechanics
to come up with nothing.