Tracks stopped halfway and next track started

I'm playing a huge playlist from a vfat partition on an external harddisk. Often, a song just stops somewhere in the middle (but not every time at the same moment in the same track), and the next song from the playlist is started.

Sometimes everything goes fine and I can listen the whole day to music without noticing this. Other times it happens with every track. For example today, it happened a lot while also printing. It also happens when I'm not printing, and even when I there are only programs running in the background.

I tried increasing the buffer size to the maximum (20s I think that was), but that didn't seem to change anything. Decreasing the buffer size also didn't change a thing.

I also used the 'renice' command to set the priority to the maximum (-20), and that seemed to reduce the problem a lot, but it still exists.

Note that I'm not having performance issues with other programs, and that under windows I never had similar issues. Under linux, I did have issues with Rhythmbox when playing music from an external hard disk (at a given moment it just stopped playing and I had to restart the application).

Sometimes, the following error can be seen in the terminal where I launched songbird (I'm not sure if it's every time a track has stopped):
** (songbird-bin:11281): CRITICAL **: gst_ring_buffer_prepare_read: assertion `buf->data != NULL' failed

Some info about my configuration:
- Songbird 1.2.0, Build 1146 (20090616030029)
- Songbird add-ons: DOM Inspector 2.0.0, JavaScript Debugger 0.9.87.4, Last.fm 0.4.2.1161, mashTape 0.2.9.1146 and SHOUTcast Radio 0.7.6.1161
- Ubuntu 9.04.
- linux kernel 2.6.28-15-generic
- soundblaster X-Fi, running with the 1.0 source version from the creative website

Note that I'm mostly running firefox next to songbird (3.0.15 at the moment, but the problem existed with older versions as well). Probably the issue also occurs when firefox is not running.