litter tray mishaps

Hi,
I have had my Tom for a few months now. He has been completley clean but the last few weeks he has been defecating upstairs. He does it about every 2 weeks, and did it in front of me today in my bedroom. He always urinates in his tray and his stools are fully formed. Can anyone tell me why this could be ?,
thanks
Rachel xx

Do you just have one litter box? If you do,try having him another box for his poo business. Some cats won't do both in the same box.

thanks, will try that. It's crazy because he doesn't do it all the time. about once a week-2 weeks. other times he is totally clean x