Unwanted Black Lines on Display

After reinstalling Win95 i get unusual black lines covering sections of text or graphics. These disapear when the screen is refreshed but will apear again in new windows. I have tested with other monitors but this is not the problem. I have seen theproblem on a number of PC’s now and therefore do not think that it is a particular faulty graphics card. Any ideas, suggestions comment would be helpful. Cheers.