Windows 7: BSOD: Laptop didn't sleep, which driver to blame?

Ok, this started a month ago when I was travelling by car. I closed the lid of my laptop, slipped it into backpack and put it in car trunk for 5 hr. long trip. When I opened the trunk after 5 hours, I found the laptop bag at burning temperature. Apparently the laptop failed to sleep/hibernate and the fan kept running, and since it had no air to breathe in backpack, it just kept getting hot till it ran out of battery. Whew.. a bad experience for laptop and heartache for me. I rebooted the laptop after plugging in and all went fine then on, till today.
I have reformatted and reinstalled Windows 7 ultimate since the first incident. Same thing happened today. I closed the lid, but the fan kept running. I opened the lid, but the screen wont turn on. Reclosed the lid, and reopned, and I see the same old BSOD with a message:
STOP 0x0000009F DRIVER_POWER_STATE_FAILURE
I rebooted the laptop and its running fine. I understand this has something to do with some driver not letting the sleep/hibernate process run smoothly. But this doesn't happen often. So how do I find out which driver is the culprit?

Some background first...

"Sleep" is not the same as "hibernate". While a machine is sleeping, some hardware components are still active and powered up. In particular, the RAM is getting fed with energy so that its (volatile) contents are not lost. That's why your laptop grew red hot during its time "sleeping" in the warm air-tight laptop bag. You were lucky that time. Since everything worked well afterwards, I doubt the machine sustained any permanent damage.

Hibernation is the act of dumping the RAM contents into a designated file on the hard disk and then literally shutting down, completely. On bootup, Windows reads the contents of the file back into RAM and then resumes where it left off. While it's hibernating, a machine is completely OFF.

There's a third option confusingly called a "hybrid sleep" which causes "sleep" first, followed by "hibernation" if there's no attempt to wake the machine up within a given interval.

How your laptop behaves when you shut the lid - whether it sleeps, hibernates, or shuts down - is configurable through the "power" control panel applet (from memory, I'm not on a laptop right now).

None of that has much to do with your BSOD though. To troubleshoot that:

1) Make sure the BIOS and hardware drivers (NIC, chipset, video...) are all up to date.

2) If the same 0x9F crashes persist, zip up the contents of \Windows\minidump and upload them here. (You'll need to first copy them to another location on your machine before you can zip them up.)

Thanks H2SO4, I will sure do that. I am crossing my fingers through that this wont happen again. Thank you for help.