It's likely you have DirectSound's (output plugin) fading in/out enabled.
Go to General Preferences > Plug-ins > Output > DirectSound > Fading > choose "on end of song" and uncheck 'enabled.' Click Apply and OK and play a new song. Your problem should be corrected.
*You may also want to disable "on start," "on first start" as well.*
hope this helps, jmat
[edit] ...too late
, and yes I would also advise you to upgrade to at least 5.08d or e ... have fun [/edit]