G4 iBook will power up to a restart screen

I just purchased a G4 iBook and im having quite some difficulty with it the person I bought it from took it to the mac geniuses and they ended up flashing the HDD and getting rid of the operating system so now im in the process of trying to install a new one but thats not the issue that is really impeding me the problem I am having is that not all the time the system will boot it sounds as if its starting and it chimes just like any other mac but not all the time the screen will turn on. and then while in the process of getting the screen to be some what functional half way through install the OS the screen will just go black as if the system is going to sleep. The person I purchased it said he was having problems with it randomly falling asleep and he said that by turning it back on after a hard shut down and leaving it for 5 minutes the system would work fine. But as of me having it I'm just having difficulty. Any ideas would be greatly appreciated.