Not that I’m suggesting these to resolve your problem, but have you tried to dust off your gear during the past 5 years? If not, there’s a lot to clean inside your Mac Pro. The GPU alone gathers a lot of dust, not to mention the mixer and its I/O cables. Was everything working fine before you upgraded to Mavericks? Have you upgraded from Cubase Artist 7 to full version Cubase 7.5 three days ago when things started to act up against you? Maybe some setup/preference thing isn’t up to par for ultimate performance as these are two different programs. Have you checked everything? Just asking’…
Good luck.