wp_cache_full_preload_hook issue

The wp_cache_full_preload_hook is non-repeating. The problem is if I set it to run every 24hrs, the time will shift.

Eg Mon 12am, runs for 1hr
Tue 1am, runs for 30min
Wed 1.30am, runs for 2hrs
Thu 3.30am, etc

Then eventually it shifts to daytime when actual users live

http://wordpress.org/plugins/wp-super-cache/