I don't think any of our developers have access to a system with optirun. I could be wrong though.
Does Supertuxkart freeze when you don't use optirun? Where did you download supertuxkart from? An ubuntu package or from sourceforge? Which version are you using?
If STK stops freezing when you don't use optirun, there may not be much we can do - it could just be a bug in optirun.