Links do not Always Stick

Highlighting text and using the link button to create links does not always work. Sometimes the link will not stick after saving or updating.

I have searched for other threads on this topic and found that this type of problem can happen for various reasons, such as link syntax, corrupted cache, browser used, etc. in older versions of WordPress.

Thus, I have ensured that link syntax is correct, I have cleared my browser’s cache, I am using the latest version of Firefox (17.0.1), I have deleted browser cookies, I have logged out of WordPress and logged back in again, and I have repeated the problem on another blog (same computer and hosting account though). I am using the Twenty-Ten theme. I am running Windows XP, SP3.

Here is what happens:
I create text in a post. If I highlight part of the text, I can create a link, no problem. If I highlight the entire line, the link will be created, but the text will be underlined without a color change. Updating the post does not change anything.

If I highlight several full lines of text (not wrapped text, but individual lines), the link will be created, and text will be underlined without a color change. Updating the post will remove the link on all lines. A blank line will be added above and below the text.

If I enter text so that it wraps to several lines and try to turn that into a hyperlink, the link will be created and underlined without a color change. However, text color will change after updating the post. Also, a blank line will be added below the wrapped text.

I have noticed that an extra invisible character is added at the end of lines (reminiscent of a paragraph mark in MS-Word because it seems to store formatting data), thus if a line is highlighted and made into a hyperlink, rehighlighting the line to check your link may show no link attached. This is due to highlighting the invisible character, which can be unhighlighted by holding down the shift key and pressing the back arrow once. Apparently, the invisible character cannot be part of a hyperlink.

These results are not consistent, especially when switching to different themes, but by playing around with links and various forms of text, these problems occur.

Can anyone help with this?