Don't write off the cat. conv. so fast, I had a brand new one that musta been dropped in shipping because it was toast right from the start. Symtoms..wouldn't rev over 1500-2000
and at times wouldn't go past idle, the broken chunks acted like check valves and sometimes a passage would open up and it would run half-*** decent, other times it would block off the exhaust flow almost completely. If possible try and remove the cat and give it a shake, any rattle is bad.