I am using a yun.
Today i used a serial connection.
What happend is:
When i start the board with the program the clock goes to slow its counts the seconds ones up every 7 seconds.
If "LightOn" the light goes "on" and the clock changes every second, so thats oke.
If "LightOff" with a value 50 on dimming1 and 2 to clock goes slows down.
So i think there is a relation between the clock and the dimmer value.