Sometimes the car will start, sometimes it won't...

I have a 1997 Lincoln Town Car.  For the last few weeks I have had problems starting it.  
Ir used to be that if I held the gearshift in Park, it would start.  Then the lights and bells,
etc., stopped coming on in any position on the ignition.  Suddenly, lights, etc., come
on and the car would start.  Sometimes only when I would hold the gearshift further up
into the park position but not always.  Now, even when the lights are on, nothing
happens at all.  It simply won't start.  I have heard a faint clicking noise, sometimes
just one loud click.  The battery is not the problem as I had it tested.  Help.......