Sorry to hear about that! Are you using a USB cable that you received from us?
We are able to reproduce this problem here when using "thin" usb cables not rated for fast charging (even very short ones, like 20cm). The picture from the camera sometimes shows up for a while, then Photobooth reports "no connected camera", then the video comes back, then black screen for a while, etc. Using the correct cable with the same JeVois unit and same Mac yields good and reliable operation.
Please see here for more info about cables: http://jevois.org/doc/UserConnect.html