When you say run for a few seconds, how long time is that? In the configuration of MD by default the Trig Duration is set to 10s. This mean that when MD is triggered it will start recording for about 10 seconds. The file will be about 11-12 sec long since the pre-trigger buffer is about 2 seconds. If the MD is triggered again during this 10 sec the recording is extended. So vary that parameter and see what is going on.