Safari and Tokens

I seem to be having occasional problems authenticating when I log into my wp-admin using Safari. After the login ID and PW are provided, rather than the one-time challenge screen, I get an error message stating that the token is invalid. I’ve tried clearing the browser’s cache and cookies but still get this error message. The frustrating part is that maybe 25% of the time I can log in so it’s hard to troubleshoot. Logging in via Chrome is no problem. Ideas?