I'm afraid I haven't found much rhyme or reason to it. It seems to have something to do with graphics cards and is more prevalent on Macs. I used to think increasing RAM would help, but for some users that hasn't had an effect either, so I'm beginning to doubt that helps.
I think closing other background applications is a good idea, but realistically it is something either the Dreamweaver team will have to fix or we will have to find a better work around for in future versions. Sorry I can't be of more help than that for now.