The first thing I'd try is another litterbox. She's not that old, so this behavior may just be a growing up thing (as opposed to an adult cat that was fine for years and is suddenly doing this weird thing...then I'd say, go to the vet).
Some cats just prefer two boxes, one for each bodily function. They don't like to poop where they pee. You can put both boxes in the basement and see if that helps.
If it doesn't leave the old one downstairs and put the new one in the bathtub. If it still doesn't work, I'd check her out at the vet.
Since she uses the one you have (although sporatically), I'd assume that she likes the shape/style of the litterbox and the brand of litter that you are using. So, I wouldn't think you'd need to start experimenting with those things.