Outgoing email keeps defaulting to gmail

For the last several weeks Apple Mail, my primary mail program, keeps defaulting to my seldom-used gmail address for outgoing messages. Both new messages and replies randomly show my gmail address in the From: section, even when I'm replying to a string of messages that started with my me.com address. In Settings, the "Send new messages from" is clearly showing my me.com address, but on occasion it's not even an option to choose when sending mail. I don't know how to override or fix this. Can anyone help?