Jamie and I were doing some locking, and we found that the Yarm green wasn't locking. It would flash, but not really stay locked for more than a few seconds, and sometimes the green light would totally disappear. If the end shutter is open, you can always see some green light on the arm transmission cameras. So if the shutter is open but there is nothing on the camera, that means something is wrong.
I went down to the end, and indeed, sometimes the green light completely disappears from the end table. At those times, the LED on the front of the laser goes off, then it comes back on, and the green light is back. This also corresponds to the POWER display on the lcd on the laser driver going to ~0 (usually it reads ~680mW, but then it goes to ~40mW). The laser stays off for 1-2 seconds, then comes back and stays on for 1-2 minutes, before turning off for a few seconds again.
Koji suggested turning the laser off for an hour or so to see if letting it cool down helps (I just turned it off ~10min ago), otherwise we may have to ship it somewhere for repairs :(