Output video is 2 times as long as the input

Hi - I’m having the same problem with a couple of dvr-ms files of recorded TV that I’m trying to convert to either mp4 or ts. Other files of the same type have converted fine but two have significantly increased in size and reported length - from about 1 hour to about 2 hours 40 minutes. The playback audio of the converted files appears to be fine but the video stutters hugely. Has a problem/solution been identified? I can supply a before and after file and the log file if needed for investigation. Thanks