HD playback goes haywire

[jeff markham]

hi,
i've got a "so small" hd-1 running on a athlon 64 x2 4800+.
when playing back sessions with 4 or more tracks, at some point,
the tracks fall out of sync and appear to come in near random
order. increasing the playback buffer size makes it better .. but
the time at which it falls out of sync just increases .. (like from
8 seconds at 1024 to 20 seconds at 2048).
i can't figure out what's going on .. PCI latency issue (like the CORE
card isnt' getting enough bus cycles)? is it just that i'm running
athlon rather than xeon? ;-(

thanks,
jeff