I had a truck come into my work one time with a kind of similar problem. The cat had melted into a couple big balls of crap internally and when you got onto the throttle hard it would perfectly plug the outlet of the cat, or if you went up hill it would roll back and do the same. It was a V8 with a cat on each bank so it was only killing half the exhaust flow. After awhile I isolated the noise the chunks were making and figured out what it was. It had blown intake gaskets too which didn't help matters much.
Andrew