PS7 Problems - lines in Multiply blending mode

I've seen this asked before, but I can't find the post. In PS7, when I use the blending mode "Multiply" and lower the opacity, I get vertical lines across the image. It's really annoying. I haven't had this problem up until recently. I'm wondering if there's a setting or something that's causing the problem that maybe I'm not aware of. Can anyone help? Thanks!