Pontiac fuel system

I have a 98 pontiac Bonneville that will fall on its face and die if I floor the throttle and hold it there. If I let off it will cleanup and run smooth. Before this it would run down the hiway on flat roar contour and cut out every so often. I can put it in low gear and take off up hills ok and run it through the gears as long as I don't have to floor it.