Mac pro 1,1 fan issues

Question

Good day Mac Pro users/technicians!!
So, a quick overview of my hardware, then of my problem. My Mac Pro 1,1 has 6 GB of RAM, the latest edition of Snow Leopard, and a brand new ATI Radeon HD 5770 graphics card (formerly an nVidia GeForce GT 8800 for Mac, that "burnt out" a couple months ago from excessive use, I guess).
Through the past 6 months or so (I think it's been that long), every once in awhile, when I attempt to sleep the computer, everything appears to spin down properly, the screen goes dark, and then within a second or two, it wakes itself, and the Mac Pro's primary fan spins up to max speed and STAYS AT MAXIMUM SPEED INDEFINITELY! Functionality remains normal, but I have to "hard" shutdown (not just reboot) the computer to get the fan to stop. The fan NEVER spins up to max speed (except briefly at startup, or sometimes the GPU fan spins up during graphics intensive processes), ever, except when I try to sleep the computer (or when I leave the computer on, and it attempts to sleep itself according to my energy saver preference). This problem used to occur only sometimes. Now it occurs almost everytime the machine is slept. As a consequence, I have to leave it on always, or shut it down completely, otherwise I'll arrive at my office/awake from sleep to the sound of a vacuum cleaner beneath my desk.
I researched Google as best as I could, to find other examples of this problem (as well as this forum/community). There were many people who reported incidences of unexpected fan-spin-up on older G5s, some other 2006 Woodcrests, and 2008 Nehalems, but none that I could find for the same reason or same duration (endless!!) as in my case.
THINGS I'VE TRIED:
1. I have tried the SMC reset (shut-down, remove power cord, hold power button for 5 seconds).
2. I downloaded a temperature monitor application to check for bad heat sensors, and everything appeared OK (which makes some logical sense to me, since I could run the computer undlessly without experiencing this problem unless I try to sleep it -- if a processor core/other heat sensor were bad, I presume that fan would be spinning up whenever the computer is on)
CPU Core temps range from 30-36 deg. C
Memory modules from 54-58 deg. C
The highest reading is from the "Northbridge Heat Sink" which peaks at 65 deg. C
The lowest from the "Ambient Air" sensor at 20 deg. C
3. Removed all peripheral devices prior to startup, but still able to reproduce problem
Can anyone suggest a course of action, besides something drastic like re-installing the system? Does this sound like some kind of hardware failure? I can always give up on sleeping the machine entirely, though it is quite convenient to be able to leave it on when I've got something running, and trust that it will go to sleep on its own without waking up the neighbors, or burn out it's fans from prolonged usage when I'm away.
Thanks for your help!
-saucony_jabroni (enthusiastically coddling his 5+ year old computer into the next decade!!)

Answer 1

Thanks again, Hatter for your help. Unfortunately, I'm still experiencing my problem. I don't use a UPS, since I live in a suburban area where we rarely lose power. I DO use a Tributaries brand surge surpressor, with indicator lights that confirm the device is protected and grounded. I'm not sure how I would test that my Mac Pro's PSU unit is faulty.
I attempted to run the Apple Hardware Test, and failed at that as well, for a reason which is off-topic (Maybe this part of the discussion should be moved to another forum). Sometime after installing Snow Leopard, I decided it would be a good idea to set up a firmware password on my Mac Pro, to prevent anyone from tinkering with my system. Unfortunately, doing so prevents booting from the OS X Snow Leopard install DVD. I spent a couple hours last night trying to disable my EFI password temporarily, and was unable to do so. According to Apple's instructions, the user must reboot their computer while depressing the Option key to pull up the EFI password prompt. When I do that, my Mac Pro appears to start the boot process normally (all the ordinary spin-up sounds of hard drives/fans/etc.), but the monitor never illuminates, and the boot process seems to stall. If I press the power button to re-boot, the machine shuts down immediately, without requiring a "hard" reboot. Is it possible that my EFI password prompt DOES come up, but ISN'T being displayed on screen? I have noticed that since I got my new graphics card (the ATI 5770), my screen remains blank right up until the end of the boot process. Maybe the updated OS X boot ROM doesn't initialize the new graphics card early enough in the boot process to view the EFI password entry prompt, or somesuch thing?
I apologize for the high degree of speculation in this latest post. I rattled it off JUST before heading off to work, and probably wasn't as clear about some stuff as I could have been. If you have any ideas why I can't pull up an EFI password prompt, so that I can disable my firmware password temporarily, and run Apple Hardware Test, I'd be curious to know what they are. If you think this post should be adapted for a different forum, I can do that after work today. If you think I should just go ahead and upgrade to a new Mac Pro, I'd be happy for that recommendation as well, but this WHOLE fiasco does seem to trivial in origin for such a drastic move. This machine still runs everything SO well.
Thanks for all you do!