Setting up a new studio with Pro Tools 9. Which interface?

Is 53% CPU use really that "intense"?

No, not at all, but between the limitations of ASIO drivers, 32/64 bit compatibility + RAM addressing, and whether or not each plug is coded for multi-core support, your DAW engine seems to be taking a dump like in the FL Studio example I gave previously. Pro Tools users get DAE errors instead of this, where playback is prevented or ended during these conditions and we get these *wonderful* warning messages and 4 digit codes.

A big giveaway that this is the cause would be that it doesn't occur the first time you are pushing the DAW to that point (everything still in RAM), but once you have saved the session and are opening it from disk it becomes problematic.