This mac behaves like a spoiled child at bedtime

Have a strange problem. When putting my G5 to sleep, it will (75% of the time) start right up again within 2 seconds. Trying again results in proper spin down and sleep. Have done everything I can think of including complete restoration of HD.Use a 3rd party mouse and KB but switching back to the awful apple Apple mouse and KB and the result is the same. Any ideas? thanX

Are you running any software that polls iTunes such as Synergy? If so, software that checks an app every so many seconds can often interrupt putting your machine to sleep.