Emu XL-7 & MP-7 User's Group

hi folks. i could really use some help on this one, as it's grinding
my work to a 
halt. problem summary: my XL-7's tempo drifts in relation to an
external 
master tempo clock. yes, i have Base Tempo set to "ext". the XL-7 is 
connected to a MOTU micro lite, connected via USB to my Mac G4,
running 
Digital Performer 4.1 on OS 10.2.8.

as a Digital Performer sequence plays, the tempo on the XL-7 will
usually 
start in-sync, then jump a few BPM's higher or lower, then return to sync for a 
few measures. then it might jump 1 BPM higher, stay there for two
measures, 
and then regain sync. the fluctuations are erratic, and while it
keeps sync 
about 80% of the time, that other 20% can really screw up arpeggios,
delays, 
and LFO's that are supposed to be tempo-synced.

the problem remains whether an empty sequence plays back (no MIDI
data at 
all) or a sequence with 10 MIDI tracks and lots of CC info plays back.

i've checked, re-checked, and re-re-checked my Digital Performer sync 
settings, and of course the XL-7's Base Tempo setting.  maybe it's
the MOTU 
interface? maybe it's USB? i've heard strange rumors about MIDI and
USB not 
getting along well. maybe it's that my MIDI interface is plugged into
a USB 
hub? 

i've tried searching the archive here, but haven't found anything
like this 
problem. if anyone could direct me to a thread here that i've missed,
or of 
anyone has some insight into this problem, you would have my eternal 
gratitude. my hunch is that either i don't have DP's "transmit sync" settings 
right (even though i THINK i do), or that there is a problem with USB or my 
MIDI interface.

thanks!