While there are a few instrument setup and bow possibilities that could be to blame, I would lean towards it being a technique issue. Usually a "warble" comes from having too much tension in your arm, and occasionally too much rosin. Put the two together, and the rosin sticks too much to the string and your tense arm has to respond with little jerky motions, and you get an uneven sound. Relax your bow arm and bow hold if there is excessive tension in your arm. If it is just an open string problem and not a closed notes problem, it could also have to do with how you are holding and supporting the bass with your left hand, and the bass itself is warbling. The only difference you should have with your left hand on open strings is that you don't have any fingers down, meaning your thumb is still supporting things and the instrument isn't moving in weird ways.
When was the last time you had a rehair? Old hair can be inconsistent in grabbing the string, and you usually need to apply more rosin to get the "same" feel that you had with newer hair, leading to the problem mentioned above. Old/dead strings can also lead to inconsistency, which is another possible guess.
With that said, if the warble happens in the exact same spot on the bow every time, there is a chance you have a flat spot/camber issue. In that case you can "smooth it out" a bit with your technique, but ultimately it should be recambered by a bow maker.
Finally, I would suggest tweaking your hair tension lower, not higher. As long as the hair is not bottoming out against the strings, less tension often gives you a smoother sound. More hair tension can make louder dynamics and more aggressive attacks easier, but it pulls the bow straighter, so the camber cannot get you "into the string" as effectively.
