I had a slack as hell fuji touring frame that I rode fixed for a couple years. I had a problem with dropping chains (while riding brakeless) so I got really fastidious with checking chain tension. But sure enough if I hit a bump or rode over streetcar tracks at high speed I'd drop the chain and be on a wild survival ride. As far as I can figure it was frame flex. I got a new frame, swapped all the parts over, and problem solved. Long chainstays with a fixed gear are bad news!