Cat pooping on carpet

Hello Everybody,
My question is this: My mother-in-law has a 9 year old cat named Noah. For the last couple of months he has been pooping on the carpet in the living room . It doesn't happen everyday, so far about 10 times in 5 months, but it's enough for her to want to get rid of him. She went to the vet and physically is totally ok. The vet mentioned that it might be a mental thing with him. Before this started happening she had no problems with him. She bought another litter box and still he is pooping upstairs. Nothing has changed in his routine, she tried different food and cat litter but so far no change. This started out of the blue without apparent reason.
I've looked around in the other post's but couldn't find anything close to what is happening to her and her cat. Does anybody have any suggestions before she gets rid off him???
Thanks ,
Heike

have you viewed this thread?

http://www.thecatsite.com/forums/showthread.php?t=22301

I had read that post and will tell my Mother-in-law to place on of her litterboxes upstairs. Hopefully that will help him. One other question. Would citrus peel or citrus airfresher help in this situation??? Noah seems to prefer to poop behind the sofa the most and I don't think she wants to put that litterbox right there
Thanks again ,
Heike