I used HandBrake to encode my files. The Constant Frame Rate is under the video tab settings.looking in MKV Merge GUI and I cant see a setting for 'framerate' either constant or variable.
What encoding program are you using?
I too have the LG 555c and it just stops playing some files after a few minutes- while others play just fine... I decompressed all the headers as a previous post noted, but I cant see how to change the framerate setting.