Birthdays module causes site to go 'pop'

Hi there

This is an issue I've had for a while on http://www.adamandtheants.org - when we get a great deal of traffic in one go (i.e. 100 per sec) the module causes the site to break, even when on throttle.

The problem is solved when I switch it off in SQL Admin, then normally I can switch it back on later on when things settle down.

However, although this is a quiet time, when trying to access the module via the frontend/admin module, I'm getting a white screen with the following error:

Fatal error: Call to undefined function _birthdays_get_date_fields() in /home/adamand/public_html/sites/all/modules/birthdays/birthdays.admin.inc on line 14

Can anyone shed light on why I may have had so many problems with this otherwise cool module?

Thanks,
Stan