Formatting Problems in Word 2007

I am having an annoying problem formatting in Word 2007. When I have selected a certain line of text to bold, underline, insert bullets etc. the ENTIRE section’s formatting changes as well as other unrelated text in the document. So if I have a 100 page document – you have to go back and search where these unrequested changes have been made. I am completely starting to loose my trust in this product. Has anyone else seen this? Is there a fix?