In the modified code I set it to ignore a dust monitor once it encounters an error with it. This was to avoid slowing down the loop waiting to read disconnected dust monitors. It appears that it is encountering these types of errors at random. So dust monitors are being set to invalid and then being ignored after momentary errors. I'm not yet sure the best way to address this. In the mean time, if a dust monitor goes invalid at random, you can bring up the expert screen for it and hit start. This should start it running again.