[NTG-context] column overflow at times --- any thoughts?

Hans et al, Any thoughts on this? We are using: \setupcolumns [tolerance=stretch, balance=yes, blank=flexible, rule=off, distance=2.5em] However, once in a while we get column overflow. See page 55 on: http://msenux.redwoods.edu/IntAlgText/chapter0/chapter0.pdf Are we doing this correctly? Any suggestions?