Ahh...I know the feeling..my cat is about the same age and he started doing this for awhile. I think it was a couple of things, one was that the room that his box was in wasn't ventilated enough ( he just started one day, not a long term habit) so I think that he pooped their, even though I had cleaned it with anti smell stuff, because he could still smell his box. So I opened the window all day for him. Also I think he was upset at me for something... he gets a real attitude sometimes....So I tried to play with him some more and give him some treats.... ( bribe him for love and good behaviour!). I don't know if that helps any, I really know how frustrating it can be, but know it may not be a long term problem.
ps. Ohhh I forgot i gave him a new smelling litter too. Next up is a brand new box- in case he permi-stank his!! LOL!! Not sure if he will start again! but so far so good!