Osx G5 Sleep Mode Hangs On Restart


I have a Dual G5 1.8 with OSX Tiger. When I switch screen names and leave one logged in and then become inactive the computer will not wake up from sleep mode. When I move mouse or hit esc the screen graphics are distorted and no button or mouse click will wake the machine up. My way around this has been to click on power bottom and hold for a couple seconds then release. Once I do the screen goes blank again. From there I click and hold esc until the login screen comes up with distorted graphics. Can anyone tell me if there is a fix for me to avoid this in the future. This did not happen until I upgraded to Tiger so I am assuming it is software related.