If ALL your notebooks are corrupting, that means there is something systematic about your Mathematica or the disk the notebooks are stored on.
Quick test: reset Mathematica's initialization files, in case one was damaged and is starting Mathematica in an odd state.
See http://support.wolfram.com/kb/12464 .
Another possibility is a damaged style sheet. Determining and fixing this would probably take more time than removing Mathematica and re-installing it.
