Sleep mode self-induces hibernation after a few hours when on AC power

I have a Macbook Pro 15" 2012 running OS X 10.8.2.

The sleep mode is set to Safe Sleep (mode 3) when I checked with the terminal command. I've never changed this setting from its default.

When I'm connected to AC power, I close the lid to my computer and it goes into sleep mode with the blinking light on the front, as expected. However, after several hours of being in sleep mode it will eventually put itself into hibernation mode and the blinking light on the front goes out. When I re-open the lid, the gray screen and progress bar appear as the system loads from its disk image to bring me back from hibernation.

This is not the way things worked up until last week. Previously, when connected to AC power, I could close the lid on the computer, it would go into sleep mode, and I could leave it there for a whole weekend (with the light blinking) without it then putting itself into hibernation mode. I could open the lid after days and the system was instantly ready to go. I never once saw it hibernate in months of owning it and leaving it in sleep mode for all different lengths of time.

I called Apple and opened a case, and they told me to uncheck the "Put hard disks to sleep when possible" box in the Energy panel. This did not solve the self-induced hibernation problem mentioned above.

Other than this, the computer is working and performing perfectly.

Anyone else having this problem? If so, what's causing it?

I don't think I noticed this before I did the 10.8.2 upgrade that recently came out....but I can't be 100% sure because I noticed this and did the upgrade fairly close together.