I had a 6 y.o. female cat (had since a baby) with the same problem. Ginger began this same behavior and after two years of vet visits, kitty xanax and a new set of furniture I had to give her up. I signed her over to my Vet who found a home for her where she would be the only pet.
My vet explained that it was probably because I had started taking in and adopting out litters of kittens. About 4 years ago we moved to an area (near a large park) where people left their unwanted pets. When they started showing up in my yard I began feeding them. I trapped who I could and would have them fixed but the ones that had kittens were the ones that I would take in, make people friendly and then adopt out.
I would have to agree with the vet and though it broke my heart to have to give Ginger up, I just couldn't turn my back on the outdoor ones : (
Has there been anything going on in the house(hold)?
I agree with Savas. If there are no physiological reasons for inappropriate urination, then the issue is behavioral. Cats pee where they aren't supposed to as a way to express their displeasure with something to us. The trick is to figure out what they are displeased with and either change it, or somehow arrive at a solution that will be workable to both the human and the cat.
Is there any reason for her to be stressed? Is there a situation going on in the house that you might not think would involve the cat, for instance, perhaps, have people who previously have gotten along maybe been fighting or arguing a lot? Has anyone moved into or out of your home recently? How is the cat's relationship with the dog?
Sometimes things that we think would have absolutely no effect on a pet, do affect them adversely. I have a friend whose cat started urinating inappropriately when she and her husband started fighting and ultimately divorced. Cats (and other pets as well) are extremely sensitive to the emotions of their humans, and if something is amiss with one of the people in the home, often the cat will act out in some way to signify the stress they are feeling because of the situation. I'm not suggesting that you are fighting with your spouse, I'm just using that as an example that cats can and do involve themselves in things that seemingly have nothing to do with them. They're vey sensitive to the emotions of their humans.
Ghilly
If the cat is healthy and the peeing isn't caused by a physical problem (chronic bladder infection, etc) then that just leaves behavioral. One problem is cat urine has a distinctive "marker" smell which the human nose can't identify after cleaning, but the cat still can. Use a product like "Nature's Miracle" to clean the area and make sure that marking odor is totally removed, or kitty will still smell that as a bathroom spot.
The most obvious thing I can think is your cat finds your choice in litter lacking. Some litters are either very dusty leading to respitory issues or have additives that smell/taste terrible to kitty (when bathing litter will end up in kitty's mouth...leading to a negative association with the box).
Have you tried switching litters yet, to a non scented high quality clumping litter?