What causes these rendering artifacts in my sculpted prim?

What causes these rendering artifacts in my sculpted prim? Here's a perfect example, seen from 3 different distances. The surface shimmers and changes as I move nearer, or farther. I've attached 3 PNG images.

I made the hat in Sculptypaint, and maybe 50% of the time, it looks perfectly fine in SL. But sometimes, it looks awful, as if the mesh has some weirdness on it. Sometimes, quitting and returning makes it go away, but today, it's persistent -- every time I log in, it looks the same. 2 other hats I've made do this, too.

I've seen this problem in the 2 1.19.x viewers I've used since I joined SL in March '08. These shots were taken in viewer "Second Life 1.19.1 (4) Apr 2 2008 12:03:24 (Second Life Release)"

It looks like the hat is too thin. When the LOD changes, the interior starts poking through the exterior causing those errors.

Thank you. I'm glad it's something I can control. That's great news.

i've noticed some effect too at a non sculpted prim.
i guess it appears when the textures is set to blank.

Now that's an interesting idea. I'll experiment with that.

Meanwhile, to deepen the mystery: I've just logged in, and it's not happening. Same client, computer, everything. Now my hat looks perfect, zoomed in or out.

Later the same day... it's happening again, even after 3 restarts of SL.

BUT... your tip about using a texture seems to work just fine! Thanks very much!