Cubase performance problems (mac)

Hey everyone. I’ve recently made the switch from logic and everythings’s been just fine except I’m having performance issues which really ruin my workflow. I get constant random audio dropouts and glitches which eventually lead to a complete loss of audio signal. I then have to plug out/back in my audio interface to get things working again. This behaviour does not seem to be related to number of tracks/plugins. It happens both in my orchestral template (99% of which comes from a slave pc via vepro) and in much smaller arrangements using only 1-2 vst instruments with only libs located on the mac itself (for eg soundiron’s emotional piano).
I’m running cubase 6.5 on a mac mini server (quad core i7@2.0 ghz,16 gb ram,standart hd,10.7.4). The interface is a duet (1st gen). I’m running everything at 256 buffer. Increasing the buffer to 512 seems to help but the latency is too big to play the instruments. Cubase asio meter stays at about 40% in larger track count projects and at about 15-20% in a project containing only a single midi track routed to a single vst instrument,no fx-nothing. That is even when the project is not playing. The meter then spikes on playback and I get dropouts which lead to signal loss. Any ideas what might be the cause of trouble?
I’ve attached a screenshot for clarity
Screen Shot 2012-08-19 at 1.16.06 PM.png