is it a standard terrain or an infinite terrain ?..most all of my Vue versions did things like that when using an infinite terrain til I went to a quad core, a 1 GB GC, & 4 GB of Ram. but all OS's performances can vary of course.

installing the newest update for Vue might help, tho....or if it didn't act up before upgrading to 8.5, then you may just need to try uninstalling the last update (go to 'file/cancel last update')....

or..u may just need to update your GC driver...if you're using a tablet (I do), you might want to try updating the drivers for it,..during one project I was using a standard terrain n the terrain literally exploded when I tried working on it in the terrain editor, til I changed my tablet settings.(u can go to wacom properties/options/..try checking or unchecking 'adjust the pen pressure for compatibility' ..I leave it unchecked on my wacom).