I was just about to, I went to open the sim that I was having problems with to check the bots that were doing this, and suddenly they've started dying in huge quantities...
My computer does some strange things sometimes... the problem has appeared to fix itself but I haven't changed any of the settings.
Well... I've ran it for a few tens of thousands of cycles and nothing's changed. But I'll post it anyway.... Seeing as I haven't changed any settings the settings are exactly as they were when I was having this problem. I just need a while because I'm doing a virus scan and hardly anything will work at the moment (or at snail's pace), so I'll post it in a couple of hours.