Please help.

I use Photoshop 7.0 and over the last few days it's been acting up. Whenever I try to use the blend mode Multiply the layer I'm using it on goes all liney like this. The more I lower the opacity the more prominent the lines get.
It doesn't do this to any other blend modes, just Multiply. It's getting really annoying, does anyone know how to fix this?
Please and thank-you.