strange de sync when playing dvd

I've noticed (and also some users of my builds)
that DVD playback is no more smooth as some time ago.
It's not really a de-sync since the audio will be still
in sync for all the movie, but often audio and video are
not perfectly in sync, it's like mplayer loses then
regain the sync after 1/2 sec.
I have enough cpu to decode and mplayer doesn't
says that I have problem playing.
The desync appears also when passing ac3 directly to my
external decoder (that is very strange).
All others movies are ok, vlc is able to sync audio and
video in dvd (but perhaps has ac3 passthru broken)

There is something I can do to find the source of the problem?
it may be my patches to mingw headers to have
big files support? (very unlikely)

Regards

-- 
Gianluigi Tiesi <[email protected]>
EDP Project Leader
Netfarm S.r.l. - http://www.netfarm.it/
Free Software: http://oss.netfarm.it/