Thanks for the info guys. I'm getting the shimmering from the texture itself. An extreme example would be a texture with a black background and white gridlines. For a texture like that I'd have to drop the levels so the white becomes grey (say 0.5 instead of 1.0) and gaussian blur it, I can also make sure the gridlines aren't too thin. Even after doing that I'd still get some shimmering.
I switched to trilinear by doing this:
...and it made a pretty big difference, but some lines still wink in and out a little.
I'm making my mips like this:
Am I on the right track using gaussian blur to prefilter ? Is there another filter that will eliminate only the high frequencies, so it won't blur everything ?