Hey Spool and Marc,
thx for ur tips.
U are right it depends on a whole bunch on various things to run with 64 samples. I have a new Imac with the fastest I7 and a fusion drive and it works...
The RME is connected via Firewire and as u told me i increased the Samplerate up to 128 and then to 512. Nothing
Next thing i did was to change the usb port (i put it on a belkin usb hub, without anything else connected) and changed the usb output settings.
Configuration was 3outs/1in and usb1 for surround...No plan why i had the surround channel on.
I switched to 3out/no in and surround off.
The result was that the clipping and glitching on automated parts decreased to 95%
After changing the usb port again i had the same clicks and glitches.
The funny thing about it... when it was on the usb3 port on the mac (which was a solo port for it) it clicks and on the hub (usb2) it´s reduced to 5%
I will test it today again and again, and will post my results here ... maybe it will also help someones else