Mesa,
Not being able to extrude a boss probably means you're sketching in the assembly, and not int the part. In the assembly, right-click on the part you want to edit and select "edit<partname>". This is editing in-context. In the context of the assembly, that is.
I don't mean to cloud things up here, but I found in my benchmarking that I got a substantial speed increase when I upgraded the old 53.03 drivers. Now then, I expect I'll have to revert to the certified version before I can get help from my VAR. I'll deal with that when the time comes.
Anyhow...
You can tell SW not to load the external references. I forgot where. Search help for external references and you should find it.
Do you use a lot of in-context references? I have found that I occasionally create a circular refernces without realizing it, and everything slows to a crawls.
Do any parent's of the component's plane have in-context references to the assembly? It's usually that sort of circular reference that gets me that error.
Other times, SW is just confused, and I have to delete the offending mates and re-create them. After that, it behaves.
I can't guarantee...
If the part is referenced by an assembly that you have open, then the part is kept open by the assembly, even if you don't have it open as a separate file.
Once you close the part document, yo uneed to reload the part in the assembly. Right-click on the part in the assembly, and you should find...