You may have bent pins when pulling the cpu out, very carefully remove the cpu and heat sync and look at the bottom of the cpu for any bent pins, you may be able to bend them back (use extreme caution), then put the cpu back in.
Some OS related signs of bent pins are: incorrect reported ram, incorrect clock speeds or crahses...
If every pin looks ok, you may just have a heat related issue.
Here are some examples of bent pins:
Sometimes it helps to hold the cpu this way and peer through the rows to catch any that are slightly bent: http://www.rlocman.ru/i/Image/2010/09/29/image001.jpg Rotate the cpu 90 degrees to see all rows.
The old fashion bic mechanical pencil might help fix bent pins: http://www.maximumpc.com/files/u112496/bent_pins.jpg
Just note that you risk breaking a pin off if you bend them at all even trying to bend them back. I have succesfully bent pins back a few times but those generally were not terribly bent. It really depends on how bad it is.
Edited by zingo156, 07 January 2015 - 04:32 PM.