I have regular sporadic problems bringing my Windows 7 64-bit notebook (running in a domain) to hibernate state.
About 50% of the time it goes to hibernate within 2 minutes and resumes properly. In other cases it takes more than 5 minutes and I observed cases, when it finally shut-ed down, but at restart told me, that it was not shut-down correctly.
I found this promising link > http://www.computerperformance.co.uk/windows7/windows7_sleep_problem.htm.
But in my event-log I found no entries about problems to enter sleep-mode or hibernate.
I find only a lot of Event ID 41 entries, but none of the events mentioned by Guy.