Could be an intermittent fault on the screen, its connector or the mainboard. Could be corruption.
The first is the most likely (especially if you've dropped the player)and least likely to be fixable economically.
The second might be fixable by wiping the drive and reinstalling the software (and firmware if that's poss).
I have an iPod Classic 80gb, and just recently started having some problems with my screen. There are green and purple streaks and pixelation on the screen. Everything else works fine, and the screen seems to be having "mood swings". Sometimes, i'll wake up in the morning and the screen will look perfectly normal. Then, at a later point it reverts back to the weird green/purple tinted screen.
Any ideas???

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