Why are there horizontal bars when I scroll?

I changed the background color to black and the text color to white (in Acrobat X Pro on a Windows 7 PC).  Now, when I scroll in some documents, the text is obscured by white horizontal bars.  There were no bars when the text was black and the background was white, and it happens whether or not I enable scrolling.  However, I really prefer to read with the colors inverted.  Why are those horizontal bars there, and is there some way that I can remove them?  Thank you in advance for any assistance.