iMac G5: fans running more often

My 20" iMac fans run often nowadays - a load that was tolerated before drives fans up. No idea why. In the past Canon's crummy printer drivers caused this kind of problem with a bad print job (printing is an OS X weakness), but Activity Monitor (show all) has no bad news.

CPU temp is about 168 by Thermograph measures. High, but unfortunately not too high by the low standards of the "30% failure rate" 20" G5 iMac.

iMac G5 constant fan noise - Mac Forums has some hints. I'll try those and update here. Cleaning out the air ducts sounds harmless enough.

I wonder too if Apple, knowing the problems with the design of this machine, has tweaked the temperature response curve to lower the threshold for high speed fans.

PS. I'm posting this using Firefox 2 RC-2. I love the inline spelling check!

Update 10/8/06: Seems better. Here's what I did:

1. Checked Activity Monitor for any hung processes (none found, be sure to view ALL processes)
   
2. Reset the SMU (System Management Unit)
3. Opened up the iMac and blew out clouds of dust from vents, etc.
4. Changed performance to Highest (this is odd, but it came from MacOS X Hints).
   

I'll see it it stays better. Hard to say what worked, I didn't have time to mess with this so I did everything that seemed reasonable.

Update 1/24/2010: Four years later I noticed this again. I reset the SMU and it improved. Thanks to Google custom search I found this old post and new what to do.