When creating an eBook based on HTML, correct syntax is required to ensure the final output is consistent. While most browsers can compensate any minor issue, Mobipocket Creator will not.
If the entire text is bold or italic, try to search the following two tags and remove all style formatting within the tag.
<br style="font-weight: bold;">
<br style="font-style: italic;">
<br style="text-decoration: underline;">