Problem solved!
Solution: find brain, insert brain into head, use brain.
So I was just trying Pontifex out on my other machine at work (a P3), and when I went to try 100 passes I realised the difficulty setting right next to that was at "medium". I put it on "hard" and lo and behold, the bridge broke just like at home! It's been so long since I installed it at home that I'd forgotten about the default difficulty setting option!
So there wasn't a problem after all, except that I'm stupid 
Unfortunately it does mean that my really cool k bridge is useless, lol