My 1997 Pontiac Grand Prix Gt 3.8 ltr.isn't running right.

My 1997 Grand Prix Gt 3.8ltr.runs good for awhile then it will start boggin like it is out of gas,then it will die when I slow down to turn the corner or come to a complete stop.It will start back up and die.But it will run and go if I two foot it,as in one foot on the gas and the other one on the brake.Then when it idles in park the engine will rev up and then bog down like it is going to die, but it doesn't die,it will do that about 3 or 4 times,then it will start running great again.It has been doing this for about a week and a half.What is wrong with it,could it be from bad or watered down gas?I changed the fuel filter and the oil pressure switch,I have even put a bottle of heet in the gas.