None that I can find, although there are several anchored objects inline with the text. These layouts are pretty simple aside from those.
Try turning off Live Preflight and page thumbnails in the Pages panel. I don't think it will help, giventhe description, but it's worth trying.
What's the OS?
You might also want to try an export to .idml (see Remove minor corruption by exporting) and check to see how they perform in a new user account.
OS is Win7 64 professional.
Turned off Live Preflight etc., tried the .idml export, same problem.
It's only on these specific files that have been opened from a .idml source which came from a Mac system. I had to replace fonts since we did not own the typefaces provided with the packaged files, so I am definitely suspecting a font issue. The original typefaces were OpenType and have been replaced with Adobe versions, although the problem does occur with the OpenType fonts in place.
For now I can work in them using CS 5.5 with no trouble. When time permits I may rebuild these from scratch in CS6, as they are simple layouts anyway, although I also want to figure out exactly what is the cause. Fonts are the chief suspect even though I have them installed locally. I am going to try replacing them with completely different typefaces from newer sources to see if that makes a difference.
I would suggest the divide and rule policy here. You can go ahead and use find font to replace all fonts ( on a test file off-cource ) to a generic font and then save the file and see if the same issue occours, if yes then you have it that one of the fonts is the coulprit, then the labour of half and half fonts.. divede and rule.... if not then its time to look at other places.