Photoshop Multiply Layer Mode Trouble

Lately my multiply layer mode has been acting up in Photoshop (7.0). It's okay when I use it on 100 % opacity, but it starts developing scanlines (of varying lightness and darkness) when I change the opacity...

At 100%
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At 54%
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At 13%
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Does anyone know what the problem is? And/or how to solve it?