Feline peeing

Question
I have read so many articals on how to stop your cat from peeing all over the house.  I have three cats and my olds cat is 15 months and has been peeing all over the house for almost 12 months now.  Some days she goes in her box other days on a chair, bed, other.  I have tried to confine her to one room, reward her, give her special attention and she has been tested for infection the results were negative. My other cats have no problem going in their box.  I am at the end of my rope here.  Please help

Answer
If you have ruled out the possibility of any physical problems, then there could be something in her environment that is causing her to sometimes not use the litter box.  I myself had this problems with a cat one time, and it turned out that she was afraid of another cat in the house who liked to bully her.  When the aggressive cat was around, the fearful cat would never go to the litter box.  If you only have one litter box, then I would suggest adding at least one more, although one litter box per cat is recommended.  There is also a product on the market called Feliway.  It is similar to the pheromones released by nursing mother cats, so it can have a calming effect on timid or even aggressive cats, provided they were nursed as a baby.  It can take up to a month to work, but it is definitely something I would look in to.  

Amanda