Template reset

I’ve just set up WP 2.0.4. and configured an website with a custom template.
My problem is that at apparently random moments my website’s template gets reseted to ‘default’.
Checking the options table I see how the template folder I’m setting manually is changed to default. Both options ‘stylesheet’ and ‘template’.
Where (in the WP code) is this happening?
Has anyone had this problem?
What could make this happen?
Who’s changing those option values?