Charlie,
I've just went through the exact same problem you are having.
I was driving down the NJ TurnPike around 70-75mph when a loud whooshing noise from under the hood scared the hell out of me. I pulled over immediately and looked under the hood and could find nothing. The noise was deep inside near the rear of the engine. Anyway, I was able to drive a couple of hundred miles back home (I just wasn't able to get over 70mph no matter how I floored it.)
Long story short, I went to a shop and the mech found one of the DPFT (whatever the letters are) tubes had blown off. He said that it felt real loose when he pushed it back on and that it would probably pop off again. He didn't find anything wrong with my cat either. It popped off and I check the excellent advice here and determined that my cat was indeed blocked (it started getting harder to drive at hwy speed). I loosened the flange just in front of the main cat and my problems went away. (You must only loosen the flange as complete bypassing of the cat will cause your engine to stall - back pressure issue or something) Anyway that was my confirmation: a replacement cat was installed (along with a SVT exhaust

) And no problems with that since.
SO! Loosen the flange connection in front of your cat and see if your car runs better. Then you'll know your cat has given up it's nineth life.