Midi timing in logic8 with AMT8 usb

Hello,

I have a:
P5Be-plus
E-6600 core2Duo
2Gig DDR800
nVidia 7300GT 256MB
SATA (2 HD's)
JMicron Disabled

audio interface: RME Fireface800
Midi interface: Emagic AMT8 (usb)

with OSX 10.5.1 and efi v8.0

I use it for Logic8 with my AMT8 midi interface.
Logic and the amt8 support Active Midi Transmission, (a midi timestamp protocol)
This combination should result in a very tight midi-timing.

But...

after doing some tests I noticed that the miditiming is shifting a bit (3ms variation in time between beats) during a song.
I did never notice this in logic 5.5.1 on XP.

Could it be that efi(emu) or some other osx "hack".kext is causing this?

Does anyone have a clue?

thanks