[help] laptop wont self power-off

[NorthboundOcclusive]

weird gremlin ive been dealing with since thanksgiving.

laptop in question is a lenovo g500 ( i3-3120m, 8gb ram, windows 8/8.1 x64 home)
due to this lil gremlin, the laptop has decided it no longer wants to power off, say when ever i tell it to shutdown or restart, which in addition to being annoying, is also a problem for updates that require a reboot.

mostly what will happen is windows will do its normal shutdown/reboot routine, the display will black out, i can hear the hard drive park and stop, and the fan stops as well, but the power and battery LEDs remain lit (unless/until i hold the power button for a few seconds)

if it sits like this in the semi-powered state, about 10 minutes later i will get a DRIVER_POWER_STATE_FAILURE bluescreen o' death, which DOES NOT create any memory dumps, and the laptop will immediately reboot afterwords (again, even if it was told to shut down). and by "immediately" i mean "so fast you can barely tell it BSOD'd in the first place", if i were to disable the automatic restart on system failure, the BSOD becomes substantially easier to view, but in both cases it hangs at '0% gathering more information'.

• i've set a clean boot with all non-microsoft services disabled, and also have tried the basic levels of Safe Mode, so i'm pretty sure that a service or running program isn't at fault.
  
• power options are set as shown https://dl.dropboxusercontent.com/u/446932/Screenshots/power_opt.jpg , and i've tried disabling Fast Boot as well, again with no change.
  
• the device manager shows no errors https://dl.dropboxusercontent.com/u/446932/Screenshots/dev_man.jpg
  
• if there's any clue in the event viewer, i'm not seeing one:
  

  "The operating system is shutting down at system time ‎2015‎-‎12‎-‎06T03:28:25.568571300Z. 
  The operating system started at system time ‎2015‎-‎12‎-‎06T03:34:04.496109500Z. 
  The last shutdown's success status was true. 
  The last boot's success status was true."

• The one item that looks at all sketchy is a error in 'sptd' that reads

  "Driver detected an internal error in its structures for ."

  (exactly as written, not really informative) and it only shows up for the BSODs when shutting down from safe mode, not from a regular boot
  
• Ive tried switching batteries, running on battery power only with AC unplugged, and running on AC only with battery removed
  
• i don't have any external devices connected, even the SD card reader is empty. i do have a microsoft lifecam HD-3000, which isnt normally attached, but it is the only new device since thankgsiving, i just cant see as that would prevent the laptop from powering off.
  
• as far as bios settings go,
  

  wireless lan [enabled]
  SATA contoller mode [AHCI]
  PXE boot to lan [disabled]
  power beep [enabled]
  intel virtual technology [disabled]
  bios back flash [enabled]
  secure boot [enabled]
  boot mode [UEFI]
  usb boot [enabled]

• attempted troubleshooting the DRIVER_POWER_STATE_FAILURE specifically, but my Google-fu isnt strong.
  As stated above, no dumps are being created from the BSOD, even though they are enabled (tried Automatic and Small 256KB).
  Ive run the Windows File Signature Verification utility, all are signed.
  Ive seen a suggestion to disable Advanced Power Management, but i don't see an option for that in the bios (bios version 78CN25WW(V2.03) )
  The intel graphics driver is current for this hardware, and aside from whatever the BSOD is complaining about, i don't really see any indication that a faulty driver is the culprit.
  looking into this 'sptd' thing, looks like it might have been used by Alcohol 52% (tho Alcohol hasnt been a problem in the past, but uninstalled it anyway.
  
• just today i let windows refresh itself, which reverted the laptop to its factory defaults and windows 8 (vanilla, bleugh), and that's been a total bust as well as far as fixing this issue.

i'm tempted to try resetting the bios (doesnt seem to be an update newer than v2.03) on the off-chance that might help

thanks for at least looking over this. if someone's got a bright idea, or wants to point out something blindingly obvious that i somehow missed, please say so