Boy I am not familiar with this at all. Why are the coming on every 15 seconds, wouldn't they just come on when heat was required? From what I gather these are meant to keep the temp at a constant level +/- .15 degree. Is that why they are cycling so often? I think I would go back to the old mercury switch and let the temp change a couple of degrees before they came on.
By cycling the heat in short bursts they maintain a more even temperature. By running the heater until the mercury switch trips at the desired temperature, you end up over-heating a good portion of the room.