Horizontal Lines on my iMac

hi bought imac g5 20" model number a1207. problem horizontal line or 2 appear on screen , has been freezing.

ps, , when boot in safe mode don't appear.

it's running on snow leopard 2.16 ghz intel core duo 1 gb mhz ddr2 sdram

 

horizontal lines

have same problem. upgraded snow leopard , seems have gotten worse. lines seem worsen when playing videos of kind, , when using skype , garageband. suspect it's software glitch. appreciate how correct this, perfect otherwise!

thanks,
connie

 

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