Performance gets worse as engine gets warm

When I first start my car, it does just fine. As it warms up, performance gets worse. Terrible acceleration, very rough idle, and my mpg is around 10. Also have a bit of a clunk/pop that develops when its warm, sounds like a
tennis ball in a metal shoebox. So far, I've replaced the cat converter (which was bad, but didnt fix the problems). New plugs and wires are next on the list, but I was just wondering if anyone may have any suggestions as to what else I should check out.