Abrar, OK, your problem could be that you have a player with a fake memory chip. There are more and more reports of people buying 1GB, 2GB, 4GB players that have much smaller memory chips that have been hacked so that they appear to be much larger capacity. The problems you are having are a good indication that this may be the case.
This post by Zaidon will explain the problem for you
http://www.mympxplayer.com/viewtopic.php?t=424
I suggest that you try to format your player and see what happens.