Corrupted Files in MS4 Scores (Converting from MS3 -> MS4)

I have several scores which I created in MS3 and converted in MS4. After working on them for a while, I saved/closed them. Opened them later, and MS4 tells me the file is corrupted. I choose "Ignore" and open the file. Upon inspection, it seems that MS4 has added additional beats to a measure/measures. I correct the problem(s), and save the file in MS4. Upon reopening it, I get the same file corrupted message. This has cost me hours of work and re-editing. Anyone else having this problem? I know in MS3 there was a way to identify and fix the corruptions, but that seems to have been omitted from MS4? Any suggestions or secret tricks I should know about? TIA!