[Plextalk] Plextalk pocket not recording properly

Hi, I have developed an ongoing issue with my Plextalk Pocket which I cannot resolve. When I am recording both with an auxiliary microphone or the built in microphone, after about 20 minutes it ceases to record as though I have pressed the stop button. I can resume recording but then it repeats the procedure just a few minutes later and then carries on in the same fashion. I can confirm that there is adequate space on the SD card and that the battery has a good charge in it. Any ideas please? Thanks, Steve