It's not doing it at the moment with recent changes in my design, so I doubt it would make any difference trying on another system.
I've since thought it might be related to how long DipTrace has been running, or perhaps how many times update copper pours had been run. On the days that this was really ****ing me off, I had been running DipTrace for several hours. Although, having said that, I seem to remember quitting DipTrace and then restarting it, only to have it happen again straight away. So maybe I'm talking crap.
Maybe there was some otherwise-insignificant detail in my layout that caused the algorithm for copper pour recalculation to work itself into a knot.