Table goes out of alignment in text box

I have created a very simple table in InDesign CS5 on Windows 7, like this:

However when you start typing into a cell, it goes like this:

To fix it up you have to make the text box slightly bigger or smaller then resize back to what it's supposed to be. Also when I do a datamerge the first page shifts like this too, although the rest are correct. When you export to PDF it comes out wrong as well. If someone wants a sample file I can post it on here.

Anyone have any ideas what is causing the alignment to change when text is entered into the table?