Mac pro locking up

Question

I have a first gen Mac pro, I am getting a sporadic lock up where the screen freezes, and it stops responding to keyboard and mouse input (and it becomes inaccessible from the network). This has happened in normal use twice.
I have tried several times to run the Apple Hardware Test off the original Os X disk, but it locks up up every time in that mode. It also locks up immediately when I boot into single user mode (which I did in order to attempt to run memtest). At least it refuses keyboard input in Single User (with my normal bluetooth keyboard, as well as a USB keyboard that I borrowed).
I did run memtest from a terminal window while logged in normally, and it was able to access 11 of my 12 gigs of RAM, and it passed, so I guess I am 11/12ths sure it is not bad RAM.
When the system does lock up, when I do a hard power cycle with the button on the front of the machine, it will not reboot -- my screen remains black. If I let it sit for a minute or two like that, then power cycle again, it does boot up normally. This is very consistent on these lockups - no boot the first time, works fine the second.
In general, I think it is running hotter than it used to -- at least it seems like the fans are running at high speed more than they used to, but I can't swear to, or quantify that.
I was on the phone with Apple tech support for about an hour yesterday, and he finally decided it had to be taken in for service.
I wanted to see if anybody here has any ideas what the problem might be, especially if it is something I could fix myself, before I lose the machine for however long it will take Apple to fix it.

Answer 1

Pull your disk drives and try booting.
If the journal is corrupt, or the directory is severely damaged/corrupt, it can prevent booting.
Maintain your drives with Alsoft Disk Warrior and keep multiple backups and bootable drives. One so that you can just pop it in if needed as replacement.
Memtest: would need to run 4-5 cycles so I hope you run 40 concurrent sessions in Terminal window to give it a good shot.
OS X Leopard has had problems with mouse or USB devices not responding or causing problems. And Disk Utility is not a big help, if at all.
Ever freeze and hard boot just adds more corruption to directory and journal making it worse.
Start with a newly formatted drive, install OS X while your other drives are out and pulled. Update and backup. Take small measured steps and build a good firm foundation, backing up as you go possibly.
First gen Mac Pro does have a small SMC button if needed to reset.
Running SmcFanControl 2.1.2 is a good measure for keeping things cooler than they would otherwise.
SuperDuper to keep a bootable backup, too.