My G5 iMac also has intermittent fan speeds that seem very high. At time the CPU fan will run 4000 rpm and CPU temp over 155 F.
After I quit a site that ramps up fan speed like U-tube the fans continue to run fast.
If my G5 then sleeps and wakes up, fan speed returns to high levels. If I restart, fans are likely to return to quiet levels aprox 1500 rpm.
Should I be concerned?