I have intermittantly experienced a problem similar to others who have posted in the past of one camera would switch on and off at random. I tried several things such as changing the USB hub port it was plugged into, changing which hub it was on and plugging and replugging the cable. After some experimentation it seems that the camera is showing a sensitivity as to which cable I use on it. All my cables are the 15ft with 90 degree mini end that came with the system from OptiTrack. I am now going to label each cable and keep a log of which camera it is used with and if the problem reoccurs.
If anyone is experiencing a similar problem I would like to hear if swapping cables around helps you.