Not sure if anyone cares about this any longer, as I can see it was posted earlier this year; but I have seen this and can recommend a fix.

If you open the signature file in word and do a reveal codes, you'll notice the lines that get the extra spacing are the ones terminated by the Microsoft Paragraph symbol (the backward P). Replace this with an ordinary carriage return and the problem will go away.

The easiest way to do this is to compose the signature in a simple text editor like notepad and drop it back into the signature editor. You can even go back and add rich text formatting with Word as long as you are careful not to reintroduce the MS Paragraph terminator (Just don't hit the enter key while editing your new sig in Word).

A Womand Told Me
http://www.awomantoldme.com