Wow- it worked!
Just change the message editor interface to "standard editor" in your options. For some reason, it worked! Although it's a bit harder to use that editor than the WYSIWYG editor if you have a complicated signature, it seems to have fixed the problem for me :D
Thanks Hisari :) more helpful than mods or admin, this time! That's something you don't see very often :P they usually know everything.