unfortunately you are probably experiencing a common issue with this series - the graphics processor either fails or its solder joints to the logic board become loose.
some have had success using non-conductive material (folder paper square, etc) to create a shim between the chip and the case, causing force and pressure to hold the chip stable. others have been able to get the chip solder reflowed.
see these existing answers:
Why does my iBook randomly freeze?
Display Screen stays black, but computer still works good
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