It can be hard to start/stop the recorder at the exact moment. There are a few tricks to make this easier, one is to use a record trigger, basically a separate universe/channel that is only used to start/stop the recording (documented in the recording manual). Another option is to take the capture files (which are standard Wireshark files) and run a post-processor on the files to trim the capture data at a loop point. There is an open source project that can assist with that, see this link: https://github.com/HakanL/animatroller/wiki/Animatroller-Tools