odd behavior of the ventilators


First : sorry for my very bad english.
I'm on iMacG5 1,8GHz 1Go DDR SDRAM 10.4.2.
Some times when I turn my iMac to suspend activity, after a few seconds it was suspended, the ventilators beginning to work full speed, and the system is blocked. I have to stop the machine by clicking off the power button.
Somebody knows the same problem ? Which is the problem ?
Thank you for your attention.