The problem is with your graphics card. I haven't checked, but I don't think yours is supported. However, there have been a number of reports here from people who were using unsupported graphics cards and it worked okay, but then stopped. That's usually (maybe always) caused by a graphics card driver update. You might try reverting to an earlier driver, but I'm not making any promises. See Highlighting Edges Takes Forever -- Started Recently. Your best option would of course be to get a certified graphics card, but I don't know if that's practical for you.