Mac Pro 2008 having boot issues...

Hi,

My Mac Pro is having boot issues,It occasionally gets stuck when I turn it on and the fans go up a bit then I just get a grey screen, sometimes just shutting down and restarting works and sometimes not. I repaired the disc though it says has no issues and repaired permissions. It still does it. if I hold down the option key and boot and select my system drive then it boots fine. Once up and running everything seems fine it wakes from sleep no problem but after a shutdown it sometimes will not boot, it's almost 50 50 chance at this point that it does not boot. I can try booting off and older back up and see if it happens. I do have two video cards and 12 gigs of ram but not sure when this started to happen but it seems to be happening more often lately.

The Mac Pro warranty is over, taking it in for service is not an option at this point. I will install Snow Leopard on a new drive and see if it still happens as a test.