same story on my singlespeed 'cross bike. i'm running a 42t salsa chaining and 16t shimano freewheel. when the left crank is pointed forwards, the chain is tight. when the right crank is pointed backwards, the chain is slack. good thing is i can tense up the chain before drops and stuff. bad thing is it feels like i'm not getting all the power into the rear wheel.
i plan to fix this by getting a steel surly chainring and white industries freewheel next summer. we'll see what happens.