MacBook Pro Fails to Sleep Properly

MacBook Pro Fails to Sleep Properly

I have a MacBook Pro 17" (late 2009 model) 2.8ghz Core2 Duo running MacOs
10.8.4 which has developed a problem going to sleep which I've been trying
to isolate for the past several months. Here is one iteration of this
problem: I manually put the computer to sleep (Apple Menu --> Sleep) with
the battery at >30% last night and then after a few seconds closed the
case. When I opened the Mac this morning, it awoke from sleep and prompted
me to put in my user password (battery at 8%) and then rather than
re-enter the OS instance from the night before, it proceeded to reboot. In
other cases, the machine will fail to go to sleep at all, particularly if
I close the laptop without manually putting it to sleep, such that when I
pull it out of the case it's blazing hot from running and not being able
to cool.
I've tried a number of fixes, none of which has fixed the problem: - I
completely reinstalled MacOS onto a fresh partition and then only restored
data and documents from Time Machine, so all the preference files were
new. - I've reset the PRAM/NVRAM several times, and the PMU several times
as well (though I'm not completely sure how to confirm that the latter has
been done successfully - I've followed the procedure specific to my mac
with a non-removable battery).
Here's my primary question: is there a possible hardware issue that could
be causing my problem, preventing the mac from entering sleep consistently
or waking up with a known sleep state?