Only tells me that your SNR margin is fluctuating slightly more than I would expect, which indicates some interference. Nothing immediately springs to mind.
Have you tried looking at your internal phone wiring? There's some Low SNR tips
here which might help to reduce the interference.
Disconnecting the ringwire is a trick that seems to work for a lot of people, and on the bottom of that page there's a link to Yarwell's blog post about how to do it.
Other than that, I'm pretty out of ideas, it might be just "one of those things" unfortunately.