The Vaisala engineers determined that the cause is due to moisture getting into the service port.
If you recall, if you are plugged into the service port the serial connect is shut down.
So you always have to remember to unplug the USB cable to test the unit.
The moisture was shorting out this port and thus shutting down the serial port.
They have come up with a new more robust connector and will be installing it on future units.
