Mac Pro restarts 5 mins after booting up

[ampnerd]

I'm having a problem with my Mac Pro 8 core (2009 model) where I will boot it up in the morning and five (approx) minutes later it will suddenly reboot. It doesn't make a difference whether I'm doing something on it at the time or if it is sitting idle.

Sometimes it will start to reboot and it will get as far as the Apple logo screen and reboot again, sometimes 2 or 3 times.

Once the machine has been on for a while it doesn't do this anymore, only when it's first powered up for the day.

Can anyone suggest what this might mean? A logic board going bad perhaps?

Thanks

Sean