Maximum PC

I have the above CPU fitted to a ASUS M4A88TDM board on a self build machine.
Everything works OK and initially all 4 cores were activated. Some time later, cant remember exactly, I noticed on boot-up, it said only 3 cores were activated. Funny ? Not really bothered since I dont use max speed or max power, so I left it alone.
Recently I noticed again on boot up, that not all the cores were activated, "Press 4 to activate all cores". Which I did, and next time around it said "3 cores activated" !
What on earth is going on ? Why am I losing cores ? There must be a memory button somewhere I am not ticking . Any help and advice please to an oldie who is trying hard to keep up. Thanks in advance.