PowerBook - Black Screen (Sleep)

Helo guys,

That´s the question:

I use the powerbook with 30/60 minutes, then i get some "black screen" but the notebook still with power on (ligths on) and if i press shutdown button (i just try that) some times then come back to normal. (Sometimes i need to push of the battery to reset the computer)

That´s happens all time, and i try command + option + r + p, to reset the "PRAM".

Can someone helps me ?
Sorry about my english.