[ffooky]

If you have Toast then you can use that but as you already have ffmpegX up and running you can achieve equally satisfactory results.

Unfortunately, MPG covers a multitude of sins and it's necessary to determine the exact nature of the files that you have. You need to know whether they are MPEG 1 or MPEG 2, what resolution & standard (i.e.PAL or NTSC), what codec has been used for the audio and what bitrate/sample rate it is.

You can glean this info from ffmpegX but I'd recommend MPEG Streamclip. Open each of the files in turn and go File->Show Stream Info (cmd-I).

Copy/paste the info box for all your files, see if the audio & video properties are the same for all and report the data back here.