what OS
what lmms
what instrument?
what export settings? ..default??? try default
buffer??
sound-devise???
what export format
And it replayes perfect when you just play it??
*.xpf are zynAddSubFx (zyn) presets. zyn works perfect in lmms, but where did you get the preset from, did you make it your self, or get it from the net? -Could be that the file is made on a newer version of zyn, than the one in current lmms..??
Have you tried to export the same song with a different instrument, as test -try 3oc
does it work?