I'm doing a full build. I've been commenting out bits of the code, and it works if I comment my Prismatic joints out. Its obviously something with the way I'm setting them up - since some of the examples use Prismatic Joints and they work good . It didn't generate a problem in the previous build though. I'll let you know when I isolate the problem - it was probably something I shouldn't have been doing, but the previous version let me get away with. Anyway I'll get back to you.