This issue is resolved for me, but I am posting it in case it helps others to diagnose these symptoms and avoid a lengthy troubleshooting process.
I was randomly experiencing extremely bizarre waveform display and playback behavior in Nuendo 5 and 6. It turned out to be due to my external USB hard drive being too slow, and/or because it has a sleep mode. The solution was to move projects to a faster hard drive. This seems obvious to me now, but when this issue first started to randomly occur, it had me stumped as to the cause.
When Nuendo cannot access project assets from the hard drive at adequate speeds, it does not provide any obvious warning or error messages stating this. Or at least it did not for me. It doesn’t seem able to detect and warn about this condition. Instead, I encountered the symptoms that follow:
-Region waveforms would appear to be drawn correctly and displayed correctly (they match what I know the source material to be), but then when I playback I get silence and occasional bursts of loud digital or white noise.
-If I zoom in horizontally on the waveform, at some point of magnification the waveform suddenly changes visually, to a completely different waveform that displays the “corrupted” version of the waveform. I can then see a bunch of silence, and a few certain points where the digital noise occurs. This waveform corresponds correctly to what I hear if I try to play the region.
-When zooming back out horizontally, the waveform suddenly snaps back visually to the other previous waveform (that matches the source material, but does not match what I hear if I press play). I thought I had a corrupted project file or something like that.
-If I closed and re-open the project, it was random whether certain regions would exhibit corrupted behavior.
-Upon moving projects to a faster hard drive, these problems disappeared.