Power Mac G5 sleeps, then freezes and fans go overdrive.

I've browsed back a bit and noticed there are more than a few other people with this same issue.

I have a Power Mac G5 Dual 1.8. After upgrading to Leopard I noticed this happening more often, but ever now and then my Mac will go to sleep and freeze there. No keyboard action will wake it. Plus, while it is frozen asleep the fans to into overdrive. I have heard the fans kick up before while the machine is awake, but I have never heard anything like when this asleep/freeze happens. My wife likens it to a jet engine.

I recall this happening once through all iterations of Tiger. But with Leopard, it has now happened four times total.

I had done a clean install of Leopard, and I've only reinstalled a few other programs—iLife, iWork, Office 2004, CS 3, so no legacy from Tiger is hanging around in there.

Still, I have a habit of launching Activity Monitor when things behave strangely and I had noticed some strange things in Leopard that has me checking this out more often recently. I think I may have noticed some odd behavior by some programs that may be related to my Power Mac G5 intermittently freeze/revving unbelievably while asleep.

It seems that there is a greatest likelihood of the computer revving while asleep when the Real Memory allocated to the kernel_task process has climbed to roughly 300 MB. I thought it might also be a factor when other apps grabbed a massive chunk of Real Memory, so I've been alert for when Safari (for instance) climbs towards 300 MB of Real Memory as well. But no, the only one that seemed for certain to be related to this issue was kernel_task climbing to stratospheric Real Memory amounts.

Quiting apps helps get stupid programs like Safari to free up its Real Memory allocation, but I don't know of any way to get kernel_task to release its Real Memory allocation to proper levels besides restarting the entire computer.

Ever since I have noticed this apparent correlation between kernal_task and the freeze/sleep + fans issue, I restart or shutdown my Mac. And since I began shutting down my Mac when I see the Real Memory of kernel_task getting too high I have not had my Power Mac experience the freeze/sleep + fans.

I would be curious if anyone else who had noticed this freeze/sleep + fans issue has noticed if doing what I describe above helps them. I'm not sure why roughly 300MB of Real Memory seems to be a tipping point for me and I wonder if it might be a different amount for other people with Power Mac G5s and this issue. I wonder why kernel_task climbs to the heavens like a spreading wildfire grabbing more Real Memory and I wonder why Safari does it as well.