Catalytic convertor?

Hi all:

My 2000 Hyundai Accent, auto trans, has been acting up lately. I'll describe the symptoms and maybe someone can suggest what the problem might be. I think it's the catalytic convertor but I'm only guessing. Firstly, the car is very slow accelerating, especially uphill. Secondly, it is using up a lot of gas. And thirdly, the transmission sometimes changes gears inadvertently and very abruptly. Any suggestions? Has anyone had this problem before?

Thanks,
Brian