Objects moving to different levels of negative Z---on their own

I just thought I would throw this out there. I have been having a problem of my drawings not staying where drawn. This only happens in the Z direction—and Negative movement only. It seems to occur randomly, on a part of the drawing I’m not working on. My drawings are 90% 2D with 10% 3D, and can get to be thousands of objects/curves. Sometimes I have had 3D objects flattened to one Z level. It happened in Windows 7 Pro & now in Windows 10 Pro. My Workstation is i7 860 - around 2.4, 16 Gb of Ram, NVIDIA K600 with 2 gb memory. NVIDIA seems to think that it is an internal Rhino bug…???..I have talked to McNeel about this problem, and it doesn’t seem that it is an issue in the Rhino Community at large; however, they have heard about this before.
If anyone has experienced this, or has knowledge as to what may be causing this—comments are welcomed.

Thanks for your time…

This is really a stretch, but if you copied an object with Record History turned on, then when the original was moved, the history enabled copy would move too.

Actually, I’ve never found the need to use History. My Projects get quite large; however, the usually don’t get into complex— I would rate my projects in the Basic category. I had to leave for an hour—and came back & checked—because of your response---- Objects have moved. It happens so often, I usually don’t fix it all back, but maybe, once a day. You have to work in multiple views to get everything back in place with 3D objects. It’s not as simple as just Setpt for 2D.
Thanks for the response though …

If my guess is right, the HistoryPurge command may be helpful.

What input devices are you using? The 3DConnexion SpaceNavigators, etc can develop a drift (recalibrate to eliminate). Not sure how that would cause your problem though.