It's just the way Winamp DSP works, there's no "reset" function. Here's an old thread about the matter...
http://www.un4seen.com/forum/?topic=2513But here's a wrapper update to try, with a "reset" option that'll reload the plugin, therefore starting with fresh buffers...
www.un4seen.com/stuff/xmp-wadsp.dllNote that some poxy plugins (eg. Enhancer) will probably crash with it enabled though, as described in this other thread...
http://www.un4seen.com/forum/?topic=3652