"sometimes with some firmware he recognize the mp3 as a USB mass storage" - this is very important.
When your mp3 will be recognize as USB mass storage you must try to flash to them dump for ATJ2051 file from site
http://www.s1mp3.org/pl/downloads_fw.php
for example:
dump_2051-A2_2006-04-20.7z
dump_2051-AB.7z
dump_2051-B.7z
atj_2051w_yf00808.7z
atj2051.7z
So make your MP3 recognize as USB mass storage and then try flashing ONLY the dump ATJ2051 file.
If after flashing a dump he will be recognize as ADFU then again make your MP3 recognize as USB mass storage and try again with another dumpATJ2051.
It must work at last because somewere in the internet your version of frimware is available - i suppose version 3.x.x