My audio recordings are drifting.  I am not very techy, but I believe based on my web searches, that this is the correct terminology for the problem I am having.  I have search the RME forums and can't find any help for this. 

I own a mac mini.  I have tested this issue in Cubase and Abelton Live.

My Fireface 800 is set as the master clock source.  It has the latest firmware installed.

If I play a midi file routed to an external keyboard and record the audio performance, the audio recording slowly goes out of sync with the midi.  Put another way, if I line up the first note of audio file after recording with the first note of the midi file, as I observe the rest of the audio, it slowly goes out of sync with the midi.

I have done another test.  I recorded the same performance from my mpc2000 sequencer into three different audio lanes.  If I line up the beginning of each audio file, they slowly go out of sync with each other --- some recordings are slightly slower or faster than others.  The MPC2000 is a rock solid sequencer as far as time goes, so that is not this issue.   

I do not know what is wrong.  I do not know much about sync, word clock, or any of this.  I made sure cubase was set to external sync so that the program is slaved to my Fireface 800.

Any help would be greatly appreciated.   

Thank you.