Mac Pro restarts instead of waking from sleep

Question

Since installing 10.5.6, my 8-core Mac Pro (early 2008) has lost the ability to wake from sleep. It seems to go to sleep pretty quickly (if a little faster than before), but when I wake it up by pressing a key on the keyboard, I hear the Mac startup chime and watch as it restarts.
I've manually shut down and booted up a few times now, and the sleep misbehavior continues.
Anyone else? My 10.5.6 MacBook has no problems waking from sleep, only the tower.
--Chris

Answer 1

Well, I thought this was resolved a couple of weeks ago after reading a suggestion that called for doing a controlled shutdown, then unplugging everything including power, waiting a minute, replugging only power, and starting up, using the power button on the front to sleep and wake up. It seemed to be waking instead of restarting, and was working normally for about three weeks.
As of today, I'm back to restarting when I try to wake. I have tried the unplug-everything trick again, and have zapped the PRAM multiple times (seriously, though, does that EVER work?). Nothing helps.
As mentioned above, I'm already on the BootROM version that EFI Update 1.3 provides, so that's not an option.
Thinking the next stop might be the Genius Bar, in hopes those guys have seen this.
--Chris