Been very sluggish, having no power at all and runs so rich?

I have a 1989 f150 with the 5.0l efi/multiport injection. Recently it has been very sluggish, having no power at all, and runs so rich you can hardly breath in it. I replaced the o2 sensor in the manifold, and checd the plugs and they seem to be fine with good connecton. It has no power when trying to take off, and once in a while when driving at 55 it will free up and run fine for a couple of minutes and then just loose power again. I drive maybe 30 miles a day and use 1/2 tank of gas, also in town at stop signs it will die if I dont keep giving it throttle. It will not idle on its own whatsoever. I need to know what the next step to take on fixing this ol girl cause she is my only mode of transportation.