Logic Pro 9 Elements of Aggregate Device not in sync


After having used Logic with a PowerBook and RME Multiface/CardBus for some years, I switched to a MacBook Pro. Since it didnt have a PCMCIA slot I couldn't use the Multiface anymore so I bought a FireFace 400, which had the advantage to work on the laptop's power. I kept the PowerBook for when I needed to record more than 16 tracks at once, in that case I created an "aggregate device" with Audio MIDI Setup which allowed me to use both FF and MF at the same time. This worked flawlessly until the PB died. Recently I bought a HDSPe ExpressCard to use the Multiface again, this time with the MBP. I created a new aggregate device, connected the FF and the MF with a wordclock cable and thought I was done. But I ran into 2 problems:

First, when I boot Logic, before I can hear anything I have to reset the audio (by changing the I/O buffer size in the audio prefs). This happens every time.

Second, between tracks recorded by the FF400 and by the Multiface there is a DELAY of approximately 80 ms. Not always: the problem happened on one take of my first multitrack session, then disappeared for a while without any intervention of my part, then came back on all the takes of the last session. The delay seems consistent throughout all the takes.

So two questions:

How can I persuade my system to behave normally without having to reset the audio driver each time ?

More importantly - since the phase coherence of my session depends on it - would it be possible to realign the Multiface tracks in a more precise way than "by ear" ?

I don't rememember encountering those problems when using the PowerBook and an aggregate device, be it with FF400 + MF like mentioned before, or with FF400 + a Mackie 1620 ONYX's firewire I/O card (which was NOT locked to the FF400 since it has no wordclock connector)

All drivers are current.

Thanks for any help or suggestion !