I had a similar problem with a GameStop Xbox360 controller clone (wired).
The right stick (camera control) kept moving upwards every few seconds when not being touched.

Drove me absolutely crazy! I ended up pulling out an old Dual-Action Logitech and it's been flawless ever since.

The above posters are probably right.. darn thing is probably filthy on the inside. Maybe not cookie crumbs, but definitely dust and skin flakes. (eww)