[FFmpeg-user] Concat videos properly (without dropped or freezed frames)

[kwi wi]

Using FFmpeg on OSX, I'm trying to concat videos taken with my phone but
the final video is very messy.
The audio is off, there are frozen frames on transitions, and the last
video has a lot of dropped frames to the point where the audio starts a
good 2 seconds before its first frame.

The basic steps I've taken are as follows. A detailed account can be viewed
here<
https://docs.google.com/document/d/1ncET_V2kfKOwF8Yaro5sTjmzq_cUKmFqzAvKxYwsU1g/edit
>

I convert three *.3gp videos to *.mp4 with
  $ ffmpeg -i <input file>.3gp -ar 44100 <output file>.mp4

I create a concat text file with these contents
  file '/path/to/cloudy_skies.mp4'
  file '/path/to/morning_nwc.mp4'
  file '/path/to/snapple.mp4'

I perform the concat with
  $ ffmpeg -y -f concat -i clips.txt -c copy c.mp4

I'm getting the following messages in the concat output:

  [concat @ 0x7ffc93813000] Estimating duration from bitrate, this may be
inaccurate
  [mp4 @ 0x7ffc9385f800] st:0 PTS: 147194 DTS: 146810 < 149650 invalid,
clipping

[concat @ 0x7ffc93813000] doesn’t seem critical. Am I wrong?

[mp4 @ 0x7ffc9385f800] seems like it drops a frame every time it’s
encountered. Am I wrong on this assumption. How do I resolve this so the
converted videos concat properly?

What specs must my videos conform to in order to be concatenated properly?
I would like the final video to have no frozen frames with all audio and
video synced.

Thanks