My neutered cat, Jerry, is about 7 years old and has been spraying. His litter box is clean and I haven't noticed any other health problems. We have three other cats in the house but they all get along for the most part. This has been going on for quite some time and I don't know what could be causing it.

Any ideas??

Has he just started recently? A cat may start spraying as a result of stress. My parents cat started when he went blind. He finally adjusted and was fine until they added a dog. These were both obviously stressful situations. Whatever is causeing stress might not be as easy to pin-point but look for changes. It could be as little as a new cat in the neighborhood that he can see out the window. Try to find what could be stressing him and then try to avoid exposure to that stressor or work to desensitize him to it.