The sptd.sys may also be installed with Alcohol 120% or other applications that use the SPTD layer.
Since McAfee was involved in previous crashes I would suggest to uninstall it and see if that helps. As an alternative you could try NOD32 (paid, with a 30-day free trial) or Microsoft Security Essentials (free).
I would avoid installing any third party firewall, at this point.
Can you make available the minidump file from the last crash?