Pooping on the floor

I posted about this here several months ago:

I have a 7 year old male cat. He's had no behavior problems since
we've had him but several months ago he started pooping on the floor
in random places. He still poops in his litter box sometimes and he is
still peeing in the box (as far as I can tell). Other than that his
behavior seems normal.

He has two litter boxes because he likes to poop and pee in separate
boxes. I took him to the vet and spent $250 for them to do all sorts
of tests and conclude that he is perfectly healthy. The vet did tell
me to switch him to a high fiber cat food which I did and that seemed
to work for about two weeks and then he was back to pooping on the
floor again (today he crapped all over my couch).

And he doesn't just poop on the floor, he has to poop on whatever we
leave out on the floor, be it a magazine, shirt, book, etc.

I don't know what else to do and if he doesn't stop I'll have no
choice but to put him outside.