i5-2500K performance issue

[NelizMastr]

For some reason, since yesterday, my i5-2500K CPU started behaving weirdly. When I bootup Windows, core0 is way up in utilization, and the others are pretty much idle.
After I ran 3DMark06, I only got 2500 3DMarks for this cpu, while it should be around 6000, and it was close to 8 before, when I had it overclocked. Same goes for the CPU Queen benchmark in AIDA64, it seems to equal an Atom D2500. Something is not quite right here.
How could the performance be so poor all of a sudden? I've tried a number of things: resetting the CMOS, reinstalled Windows, checked if the BIOS was up-to-date etc.

Any guesses on why this is happening? I'm fearing my CPU might be toast.

Thanks in advance!

[fenyal]

try resetting your bios to default settings.

have you done any cleaning or rearranging of components inside the case?

what are the tempatures of the cpu when it's just hanging out?

is the cpu being cooled by water or air?

information on what your motherboard and power supply are is important to have.