Waking from sleep shuts down Mac- OSX

Foge Sretsam

Registered
Just recently if I put my iMacG5 to sleep, when I come to wake it up the machine goes dead. Nothing seems to wake it up and pressing the power key produces a high pitched whine. Sometimes after this it will start with the usual Mac 'boing', other times it needs removing power and waiting and then replugging. Any ideas as to the cause?
 
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