big crash sound sometime since OSX 10.9 and 10.10...

I have sometime a crash sound (sound destroy since 2/3 seconds), and I haven't find how resolve that 😟

I have a macbookpro 2009, core2duo, 8 Go Ram, SSD internal, sound cart external Motu UltraLight MK3 (Firewire connection).

1) Before with OSX 10.8 Mountain Lion, I never have this problem.

2) There is 1 year, I install Maverick and I have sometime (one time on 2 or 3 hour) a little crash sound (silence or crash sound since 2 seconds).

3) There is 6 month, I add 4 Go Ram to have 8 Go Ram.

No change, after that I have the same problem.

4) All Driver are update.

(On internet, I see it's a problem that many people know but without a good solution, apple don't give the solution, it's sad...)

5) I see on internet that Yosemite resolve this problem.

I Install Yosemite 10.10.1 on an external hard disk (USB connection)... and I boot on it

no change :-/

6) I change my external sound card with an RME Fireface UCX (a nice sound card, I like it)

I think at a moment it's better because I haven't had crash sound since 5 day (5 x 5h use it)...

but the 6 day, there is a big crash sound since 3 secondes. 😠

Why ?

At this moment there is : Traktor + Live + TotalMix (RME) + Digicheck (RME)

Too much prg run ?...

But I see that No overload cpu and 3 Go memory free....

Just for information :

Before with Motu Ultralight MK3 the message error (on consol) with crash (1 on 2,3 hours) :

kernel[0]: IOAudioStream[0xffffff8021ab3200]::clipIfNecessary() - Error: attempting to clip to a position more than one buffer ahead of last clip position (da55,e3c)->(da59,123c)

Now with RME UCX the message error (on consol) when I have the crash (just one on 6 days) :

(what is it the "XPC" ? and why this question "Please switch to XPC..." on logfile ?)

who will have an idea to resolve that ?

Thank you very much for your responds

JL