You could try this trick (I didn't test it - so no guaranatee is given).
As root, make fstab unwritable:

chmod a-w /etc/fstab

The next boot, you might see some errormessages, showing which script tried to write to the file - but I'm not sure whether they will give detailed erromessages.

Then you might tell us the script and the code-block, where the errors occur, and we could tell you how to change it, without breaking important things.