I recently had a new motherboard installed and replaced my 80gig hard drive which was dying for a 120 gig. My problem is, that sometimes when I boot my pc, I get an error message which says that the boot devices have changed and that the BBS boot priority would be affected. Please enter setup to check. When I do so I realise that the hard drive is not being recognized.
I get it to start working again only when I unplug and re-plug the cables.

My question is, could the socket be the problem or do I need to check the MBR?

try to swap the power connector for the hd (loose power socket), sometimes it may help. enter your bios and make your new hd first boot.

Is this an intermittant problem or does it happpen all the time? If it is intermittant, it could be a number of things, including bad sectors on the drive itself, a faulty controller or cables.

Your first port of call is to check the power and data cables, replacing them with new or known-working ones if possible. Next, try loading a set of disk utilities to scan the drive for errors.

Another possibility is that the hard drive isn't spinning up quickly enough during boot - you can increase the delay before the drive is accessed in the BIOS.

These will all cause intermittant problems that may or may not be solved by a slight movement of the cables as you have noticed.

Thanks for the advice, I switched the power cables two days ago and so far no problems.