Rev. A iMac G5 shuts down instead of sleeping

I hadn't been using sleep mode on my iMac G5 for a while (but it always worked previously to me setting it to "never"), and now yesterday when I turned the sleep mode back on, it has decided it wants to shut down instead of sleep. But it's not a nice shutdown...it just powers itself off, as if it was unplugged. It always boots back up with no problems.

Google isn't being too helpful on this issue...I did find something about resetting the SMU...might that help?

Has anyone else experienced this problem or know what to do to fix it?

Thanks