Powerpoint error

I have many PPT files stored on my dropbox and these things happen randomly to a couple of my PPT files :
They occasionally get corrupted and can’t be opened at all.
The fonts change by themselves on some of the slides. e.g. it’s supposed to be all TNR – then it randomly changes some of my paragraphs to my custom fonts.
Some of the images in the PPT file are swopped from one file to another.
Doesn’t seem to be a virus too – i did a complete scan and didn’t find anything at all.
What’s irritating is i cannot reproduce the problem; it happens intermittently.
what should i do?