xgroove~ position output wrong when using edirol FA-66 audio interface

I'm working on a DJ application that uses the excellent xgroove~ object for mp3 playback. I use the position output of xgroove~ to draw a line on a waveform display to indicate the playhead position. While using my macbook's built-in audio driver the position output and playhead behave as expected. But when i use my edirol FA-66 soundcard, the position outlet of xgroove behaves differently. It doesn't seem to advance fast enough. For instance by the end of a track that's 311823 ms long, the position output reads 143243 and the playhead marker has ended its journey somewhere in the middle of the waveform display instead of at the end (the position output seems to be 0.459374985 of the actual position).

Does anyone know what might be going wrong here, or how i can fix it? Thanks!