Car keeps cutting off in park.

I drive a 95 Buick Lasaber. For the last few months
I've had problems with it cutting off on me. I've
replaced the spark plugs and wires, the ignition
control mod, the coil packs, the gas filter and
pump. The last thing that was fixed was a screw
that had something to do with the idle had fallen
out so it was replaced. That's seemed to do the
trick for about a month. Now since the screw has
been replaced and adjusted it has cut off twice
both times in park but I can feel it trying to cut of
when I'm in drive at a complete stop.  I've spent
over $600 trying to fix one problem can someone
please give me an idea of what it could be and the
best way to fix it.