One of the biggest opponents of sugar, Dr. Robert Lustig, does not recommend the use of stevia yet, because there are no long term use studies.

However, "there are no long term studies" often is a bit of a catch phrase, can mean erring on the side of caution, and can be used against innovation.

Personally - from what I have seen and read - I doubt that it is a single cause of PN. My biggest "progress" was in fact made when I eliminated sugar as much as reasonably possible from my diet. (but I did not replace it with stevia)

You might try and stop using stevia and see if it improves the symptoms, but if they don't improve, by the nature of PN, it doesn't mean that it has not caused the PN. As an analogy: my PN was caused by alcohol, but stopping using it did not improve the symptoms until almost a year later.

Still, I would say go ahead, stop taking it for a while (and don't add back in the sugar if you can) and report back on whether you notice any difference?

Again, the above is based on my personal observations only.