Adding audio track to MKV file

[SavageButcher]

Hi everybody!
I'm a noob in video editing, but i've been trying, since a month or so, to create my own MKV files by adding ad italian AC£ track to the original english MKV.
What i try to do is:
1. Extracting the audio track from the italian dvd
2. Changing ethe framerate from 25fps to 23.976fps using either eac3to or BeHappy (i like the first one most)
3. Calculating and adding (again with either eac3to or BeHAppy) the delay to the italian track.
4. Creating the MKV file with the original video track, original english audio track, and the converted italian track.

No problem with the first 2 movies. But now i'm experiencing some difficulties.
I can't manage to get the italian track syncronized with the video: if i synch it at the beginning of the movie, at the end i always find a delay (from 0.5 sec to a couple of seconds, depending on the movie)
Why is this happening? How can i fix it?
Thanks in advance for the answers!

[Baldrick]

Maybe the movies are edited a bit differently. Then you have to find where it goes out of sync manually and resync.

[SavageButcher]

I tought so too, but the delay seems to increase scene by scene :S