iMac Intel Core 2 Duo.. Freezes up in Sleep mode

My iMac is intermittently locking up. It will go into sleep mode, usually I strike any key or move the mouse and it wakes right up. However lately it will not respond to anything. the power light is blinking slowly like it normally does, the mouse has a laser,,, so I know its getting juice. The only thing i can do is hold down the power button for 10 seconds and let it do a hard boot.
Any suggestions?

Thanks,
Tommy

Try the following procedure from the Apple KB:

Intel-based iMac, Intel-based Mac mini: How to reset the System Management Controller

Regards.