MartinS, The problems you discribe are consistent with some of the symptoms of a hacked player.
If you bought from the same seller (Keistore) you linked to he is also known to sell hacked players and appears on the ebay Blacklist.
http://www.mympxplayer.org/70-ebay-t...rs-vt3556.html
To fix the problem you can use 'lattesurfs' guide here to rebuild the players partition.
http://www.mympxplayer.org/how-to-re...ck-vt2400.html
If the player does have a hacked memory chip then you will find that after removing the hack that your player will read it's true memory size which will be much lower than the advertised size of 4GB.
Let us know the outcome.