G5 Has Trouble Waking Up


I have a new G5:
Machine Name: Power Mac G5
Machine Model: PowerMac9,1
CPU Type: PowerPC G5 (3.0)
Number Of CPUs: 1
CPU Speed: 1.8 GHz
L2 Cache (per CPU): 512 KB
Memory: 768 MB
Bus Speed: 600 MHz
Boot ROM Version: 5.2.2f2

the problem is that often when it goes to sleep it does not wake when I touch the keyboard or mouse, the only way to make it react is to turn it off with the main power button.

This happens also sometimes after I logoff, in this case, when I touch the keyboard, it may show the login screen, but once I click (the mouse does move) on a username to try to login, it gets stuck and the mouse symbol on the screen becomes the round colored wheel that means it is doing doing, but never reacts, and I have to turn it off with the power button.

I had this problem since I got the machine. Recently I upgraded to Tiger and the problem seemed to occur less frequently. Until today, when I changed the monitor: I was using a Sony Multiscan 300sf 21inch, but today I connected instead a new Cinema 20 inch, and the problem got worst. Today each time I left my office for an hour or two, when I come back it does not react, the screen off, and only way is to power it down. It seemed to me that I heard a bip or two, as when a new email arrives, so I am not sure if it is sleeping or just the monitor does not turn on again.