I also has this problem, but with picture slideshows.
I bought the DSM-510 a few weeks ago primarily to watch my holiday pictures on the TV, wireless.
I also have some mp3 music which is nice to use as background music.
I use a DI-524 router and a DNS-323 NAS unit, but I've given up using the 323 as uPNp server since it refuses to open the folders with my pictures (in total around 10 000 divided into subfolders).
So I switched to use Windows Media Player 11 as server on Windows Vista Home Premium.
After a registry patch (that I found in this forum) I got WMP11 to display the files on the DNS-323 in DSM-510. I'm using wireless with WEP enabled.
If I start playing some mp3 music as background, and then "back" and starts a slideshow with pictures the slideshow gives up after around 100 pictures and the filenames are greyed out.
The music however continues to play just fine, from the same WMP11 server, from the same DNS323 via Vista.
When I go back to the server menu, "starts the server again", aborts, and then do refresh (following another tip in this forum), the server becomes available again and I can go to the pictures and restart a slide show again. After another 100 pictures it stops again, but the music still plays on.
This is driving me nuts.
If I use my laptop on the same wireless connection at the same location as the TV it works just fine, displaying a 5 star rating for signal strength on the wireless connection.
Are there any new resolution tips available ?