you can find this setting when you have a workout open
Just click the "cog" icon on the top right, It will be in the display option
The - are shown when no data is received after a while (for this metric).
Do you see dropout on the graph? It should not behave like a 0 value, Let me know if you can show a screenshot of a whole workout.
if the ANT+ dongle is close enough and not blocked by solid object, I am not sure I can see why you are having these, did you try closing other program that may be using too much CPU?
Thanks for sharing Alex! Looking forward for more nice features we can do together with ERGdb.org, love the open source workouts!