Gary, hi! the OS on my MacPro is 10.6.8 and the connection to the Rosetta is Firewire.
I tried using the built-in analog interface and it made no difference. However, some things I have discovered.
1) this is a Midi sync problem, there is no problem on the system at 96kHz running audio only.
2) it seems to be related to CPU overload, as the CPU usage meter goes into the RED when this is occurring.
3) I often get an error message with the following: 'Error while trying to sync audio and midi. Sample rate 81859 (or 84176 or something else) recognized. Check conflict between Logic Pro and external device'.
The audio buffer size does not seem to have any affect.