Recently my G5 PPC iMac will refuse to wake from sleep and the spinning beach ball will stay for ages and the fan will go crazy.

When I use hardware switch off it takes 4-5 attempts to reboot properly, mostly it will start with the Apple sound but just blank screen with nothing.

I have seen a coupe of error messages on reboot - kernel panic?

How do I fix this...is it some software I have or hardware malfunction?

It only happened recently like last 3-4 months

try this:

1. turn off the imac;
2. pull out the power plug for about ten seconds;
3. plug the power cord back in, while at the same time holding in the power button for about ten seconds;
4. turn it on;
5. try out sleep.

This process is resetting the power management unit.

^ Interesting, I will have to try this. What is happening to the power management unit for this to be required? Is it a serious enough issue to warrant having to have my Mac looked at?

My iMac (Latest Leopard) very recently has started to refuse to wake from sleep, especially if I have left it on overnight.

There are two iMac G5 repair extension programmes for power supply issues. It might be worth checking to see if your machines are covered.

Apple - Support - Exchange & Repair Extension Programs - iMac