[V8i] 5 second 'hang' on every undo

This is a problem I had before some many months ago, but I have a rebuilt PC and the issue has returned and I forget how we resolved it.

Even on very lightweight files all functionality is perfectly fine but anytime undo is triggered either from menu, ctrl-z or toolbar icon my machine will freeze for 5-10 seconds. It makes no difference what the 'size' of the action being undone is, always a delay.

As I said, we sorted this before on Ss2, but a new machine and Ss3 the issue has returned.

Is there a size of undo buffer somewhere that will be causing this? It's baffling as it affects enormous models or simple near empty files.

Thanks