Might need to open up your Imac and make sure it ain't clogged up or maybe clean and reapply the thermal paste. :)
On Quake 3 under MorphOS, be sure to /r_primitives 2 and then /timedemo 1 .
Fans are cleaned. Next time when I will opens iMac I will check again everything. But i think it is no mechanical.
yes, all tested configurations have /r_primitives 2 and identical all settings.
AmigaOS: Pegasos II, Sam440ep-flex, Micro A1-C, AmigaOneX1000
MorphOS: Pegasos II, Powerbook G4, iMac G5, Powermac G5 Quad