512mb here as well, although the only problems I have when I start using too much memory is things get a bit slow. In my experience, it takes a fairly big model and/or many very large textures to cause this to happen,
Are entire wireframes disappearing, or just parts of them?
If only parts of them are disappearing, select the wireframe and set it to 2-sided (the "2S" button at the bottom left). One-sided polygons won't show up in 1-sided mode if they are facing "away" from the camera. Not that you necessarily want the polygons to be 2-sided, just something to check out.
And as Art mentioned, it may be a driver problem --- if the above solution doesn't work, I'd try updating your video drivers.
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