Here is a recipe for concatenating mpg and other video format files.
ffmpeg -i a.mpg -s 480x360 -maxrate 2500k -bufsize 4000k -b 700k -ar 44100 i1.mpg ffmpeg -i b.mpg -s 480x360 -maxrate 2500k -bufsize 4000k -b 700k -ar 44100 i2.mpg ffmpeg -i c.mpg -s 480x360 -maxrate 2500k -bufsize 4000k -b 700k -ar 44100 i3.mpg cat i1.mpg i2.mpg i3.mpg > a.mpg ffmpeg -i a.mpg -sameq combined.mpg
This seems to work better than simply using 'sameq' throughout.