i've just compiled for linux (my laptop) a svn snapshot of mplayer.
"dev-SVN-r26264-4.1.3 (C) 2000-2008"

This one plays perfectly flv files (a/v sync and seek)

I've noticed that ver 25 and 26 for maemo are using a mplayer version that reports in the copyright 2000-2006.
Is it using a older version?

I'm just searching for a cause why it has some problems sametime to play youtube video on my IT.

Hope this help, the alternative will be to build a stripped version mplayer that will only know how to play flv files.

Cheers
Mihai