Very Slow PC - Really Weird Problem!
To cut a long story short, I had a PC which was running very slow, not installing new allocated packages, not rebuilding properly and so on. After much messing around swapping cables and network points to rule out infrastructure problems, I got logged in and was presented an error message about AD entries, so settled on rebuilding it with a new name, and it is now happy. Good.
The hinky part is that the PC whose network point I borrowed to test the infrastructure is now doing the same thing.