After updating to OS X10.5.7 I had crashes as I had never before, unpredictable, irregular but unfortunately frequently! And viewports had a strange behavior. They showed up and disappeared again almost impossible to work on!!!
I was thinking of reinstalling my old OS because VW always worked fine almost no crash at all! Then test on an other older PB same buggy behavior ! But than SP4 turned up and with it some hope ...but still same problem. Than finally I checked my file.... my default class "none" was set to be grayed!!! (Don't know when and why but it was same in my default-file so every new file was the same...) As I was creating every element in my standard (working)class and didn't change you don't see it at the screen because it's the active class, which is alway visible.... since setting it back to visible everything is fine and smooth as always....
Just to let you all know and apologize for all my secret swearing to NNA and Apple!