Hibernate behaving erratically

Hi: I have a desktop with Windows XP updated to SP3. I always put to computer
to hiberante since I have it. A few days ago, without any configuration or
BIOS changes, the Hibernate function has stopped working. It goes to the
hibernate status but after 20 minutes (exactly) it just turns itself on
again. I have reviewed every configuration on Power Management (again, I have
not changed anything) and consulted every resource I could find, but nothing
seems to indicate this is a known issue. Any ideas?