big question: has this cat been neutered/spayed? Sometimes, a remote possibility can be that the operation might have been botched and there are still some ovaries left in there. I've heard of that happening.
Oh..somehow I thought this behavior was more recent from you're description.
I'll post some links that can help shed more light. However, I'd still bring the cat in for testing.
He's a good guy so this is Gustav's own special way of saying hello.
[Edit]
http://www.associatedcontent.com/article/31052/how_to_break_your_cats_spraying_habit.html
House cats are wonderful creatures. They love to play, they catch mice and other rodents, and they make cuddly companions. Since they can't speak verbally, cats communicate in other ways. They purr when they're content, hiss when they're upset, and they also have a tendency to spray urine for a number of reasons. If this is a problem in your house, then you need to know how to break your cat's spraying habit.
The first step in this process is to make sure your cat actually has a spraying habit. That is, are you sure your male or female cat is actually spraying? Or, does he or she simply miss the litter box? Are you finding pools of cat urine on the floor? This isn't a sign of spraying either. Normally, cats that have a habit of spraying will back up to doors, windows, walls, and furniture, et cetera. You'll find squirts of urine about a foot or so up from the floor.
The second step to break your cat's spraying habit, is to find the cause of the problem. It's a common fallacy that only male cats "mark their territory" by spraying urine. While it's true that male cats spray urine during the mating season, they do it at other times too. Female cats also spray urine to arouse the interest of a tomcat. And, they spray for other reasons as well.
However, research shows that cats who are spayed or neutered are less likely to spray than cats who aren't. If your house cats aren't "de-sexed", talk to your veterinarian about performing the surgery. Not only can it break their spraying habit, but it will help keep the problem of overpopulation down.
Ideally, all cats should be spayed or neutered by the time they reach six months of age. However, they can still be "fixed" when they are older than that. "
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[Edited for brevity]