From what I have read it does not have anything to do with boredom . It actually seems that we should not be insulted by it, it is contagious and some. It seems that many people think it has a function, possibly to cool the brain, but it really hasn't been determined for sure. There is not agreement among scholars who have come up with many theories .
I would like to think they are hungry or tired if they yawn during a serious conversation. The last resort would be they are bored with that particular conversation and wish for it to be over. Perhaps when you are having a serious conversation you can offer them a snack (not a meal!!!) and ask if they are well rested. If they continue to yawn, ask them directly if they are bored!!!! Hope this helps!!!