iMac Sleep problem

[Slrman]

I have a 2.66 GHz 20" iMac (2008 model) that will not stay asleep. Often, it will wake for no reason in the middle of the night. Some nights, several times and only seconds after I put it to sleep.

I do not have "Wake on Network Access" turned on so I cannot think what might be causing this.

Any ideas?

 

[Hellhammer]

Try repairing permissions with Disk Utility, PRAM and SMC reset

 

[Slrman]

Disk permissions have been repaired several times. But zapping the PRAM and resetting the SMC I haven't done. It will take quite a while to know if either works as this may not happen for a week at a time. But I'll try the PRAM first, then the SMC.