I've had those exact symptoms on my 01. Twice. Both times the dealer replaced the cat (warranty). Both times it fixed it. There are 2 causes for the cat clogging. 1) there is some ceramic parts where the exhaust enters the cat which break up and literally block the exhaust flow. 2) is worse. metal from the pistons, cylinders and rings wears off and eventually causes the same symptoms. I'm only familiar with #1, and do not have any verification of #2, just was told that by the dealer.