We had the problem of the theme reverting to default. The interval between changes was not fixed.
In our case it appears to have occured due to an oversight on our behalf. We had left a copy of the files in a test folder mapped to a test URL where we had tested WordPress with default settings. We had changed URL General options (for WordPress and Blog) and copied our test files to a new folder and set up our new URL and added a new theme and everything seemed fine.
Unfortunately a crawler (Yahoo in fact) found the old copy on the test URL and each time it requested the index.php the theme was changed to default (because on that URL there was no copy of the new theme and so the database wp_options table template value would have been found to be invalid).
Clearly the solution in our case was to delete the test folder (which we should have done anyway once the new site was up).
