My recently-serviced G5 was doing well. This morning it was locked up again. Fans running at full speed, screen wouldn’t wake up, couldn’t get in via SSH.
The problem: last night before I left, I went into System Preferences and checked the little box for “Put the display to sleep when the computer is inactive for…”. This was also enabled the last time it failed to wake, and based on half-remembered forum postings, I had turned it off. I feel no need to repeat the experiment again…
Looks like some problems in the power-management firmware. Which makes sense, since the problem that sent it in for service was that it went to sleep and never came back, and the amount of stuff they replaced suggested thermal issues.
Update: No, it’s just hosed. Even without the power-saving, leaving it on for a few days killed it.
Update: Ah, turning off all power-saving, including the Automatic setting for processor performance, seems to keep it alive. So, if I don’t let it sleep, don’t let it turn off the display, and don’t let it slow down or shut off any of the cores, it seems to work. I’ve been stressing the CPU on and off with a multi-threaded Sudoku generator (not because I need 200,000 Sudoku puzzles per day; it’s just a convenient way of pounding on the CPUs).
This is not acceptable in a new machine, of course, but if it does turn out to be a firmware issue, and they release a patch soon, I can at least use it in the interim.