The fans working not so good - so if you overload system, iMac probably freeze.
my iMac now operate fans correctly ( boot made by open firmware env) see here
fan is silent after boot, with high CPU usage fan runs with high rpm, with cool cpu fans again stops
AmigaOS3: Amiga 1200
AmigaOS4: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad