Problem with a v6 98 Mustang?

When driving my car, it can feel when I give it gas that the car feels like its lugging, and has this feeling of a hiccup from it. Also when I'm in a gear and I **** up to say 4th when needing to shift to 2nd instead of bogging down, it bucks really hard. It used to run great, and now when I press the gas it doesn't feel as if it's putting out nearly as much power, and takes a lot longer to rev up, and even when it revs up, still doesn't feel like enough power getting down. What do you think the problem is?