Only other thing I can think of that might possibly stop it is if you have any bent pins in the CPU socket. It is not as likely as on the newer sockets but is still possible.
Very carefully remove the heatsink and fan. Probably a slight twisting motion is best to release the two as the thermal paste can act like a glue.
Once off remove the CPU taking extreme care both pysically and electrically(static) and place somewhere safe.
Make sure that the lighting is very good and even.
Now scrutinise the socket for any pins that are out of alignment, even slightly.
If in any doubt whatsoever take a couple of macro (close up) photos of the socket and post them on here for us to check.
Also look at the bottom of the CPU at the lands (little copper circles) and make sure that there is a mark in each somewhere near the centre. If there are any missing marks or any of the pins are obviously bent then that is your problem.
