I have the same mac as you, with 8 GB also. I am using a MOTU Ultralite mk3 for audio inputs.
The only other difference is, I am not using Kontakt or Omnispherel: primarily Apple plug-ins, Toontrack Superior Drummer, and a few wave plugs. I am using a buffer size of 64, but 44.1 sample rate.
This is the most stable set up I have had, with no drop outs, glitches, crashes .... all is good. I would think you should be able to get good results also.
I suggest you take things out of the equation, one at a time, and see if you can isolate where the problem is coming from. Some suggestions:
- are all drivers and plug-ins current (especially RME)
- run MS in 32 bit if you are using any 32-bit plug-ins (the bridge can cause problems)
- if you have 64-bit plugs, try running MS in 64-bit
- try running without Kontakt for awhile, then without Ominishpere for awhile. Any changes?
- try 16-bit, 44.1 kHz. Good enough for live performance, less CPU.