Sorry, I don't know of any. I use this
It's a 12 dollar PSX to USB converter I picked up from Radio Shack. Cheap little thing, breaks easily, but it does the trick if you're careful with it. I've gone through one before, it just stopped working randomly.
I'll keep an eye out for any x-box controller related stuff and PM you or post it here if I find something.
What exactly is being inconsistent, is it the analog sticks, or perhaps input polling intervals?