have your tried to play the songs in your computer?are they corrputed in some way?have you tried to upload them to another player (doesn't matter what company) to see if they are played well?
and of course-firmware,firmware,fi rmware.check you firmware version and update if neccesary.i had this problem too.i made a scandisk to my computer and it was gone..so perhaps a bad shutdown of your machine is also involved.
Are you connecting the IRIVER via a usb hub?if so,that can cause huge amount of trouble.
perform a scandisk,reboot you machine,and check for the latest firmware.have you used files from your old player?i.e,transferr ed them to the computer when they were corrputed because of the malfunction?
perhaps when you had the old one,the files were corrupted inside of it?and when you changed it and used the files from the old one,all the files that were uploaded were corrupted?
if you can,try to boot into UMS mode and see if your player is recongnized properly.By the way,is your player UMS or MTP?
Matan.