I have seen a few people with this problem.  The one answer that people marked as "worked" was to copy and paste your signature to textedit and then back to the email reply.  That doesn't seem like a fix to me.  Especially for us that send 20-30 emails an hour.

My problem is that when I send an email and get a reply to that email, when I go to reply, my signature font changes.  It also makes the font bold and it looks strange.  It only happens on every 2-3 emails which makes it even stranger.  I've updated to the latest version and installed all patches.  I've ticked the box to maintain formatting and have tried without that option as well.  I've tried every fix on every forum for the last 3 months and nothing has worked.  Out of frustration I deleted the IMAP account and re-added it.  Waited 36 hours for it to download all my mail and the problem persisted.

PLEASE SOMEONE HELP ME!