Golden rule number 1 of troubleshooting PC...
If something misbehaves, reset BIOS to defaults/factory standards... :)
Is speedstep set by default?
Sorry, haven't used Intel since PIV.
Usually, yes. There are two pieces involved: the BIOS and the Windows power management setting. In Windows power management, "Home/Office Desk" or "High Performance" is supposed to disable SpeedStep, but does not always do so. In BIOS, disabling SpeedStep (which may be called EIST) and C1E should disable SpeedStep reliably.
Not all PCs will let you disable SpeedStep. Laptops and proprietary PCs such as HP and Dell may have those controls in the BIOS locked and may allow SpeedStep even on the highest-power Windows settings.
Generally, it is a Bad Thing to overclock unless you also lock out SpeedStep. There may be some motherboards that are stable with both overclocking and SpeedStep, but there are others that are not. You will probably be unlucky and have one of the ones that are not.