BBOD for 30 sec + after pressing play

[clausiii]

Hi!
At work we have a huge annoying problem.
We have a HDX MADI (128 I/O) system with PT10 (i tried every version from 10.2 - 10.3.7) on OSX 10.7.5 ( also tested on 10.7.4) on an officially supported MacPro with 12 GB RAM.

We mostly use is as a multitrack, around 64 tracks I/O, no plugins, each track has its dedicated in and out.
When we record orchestra rehearsals for our TV show, we get many takes with a length between 1.30 to 5 minutes. This results to a timeline length up to ca 75 minutes (48/24).
Recording itself works just fine but pressing playback often results to a Beachball which holds off PT to play for 30 sec - 1 min! After waiting it starts the playback with no error message. I think the bigger the timeline gets the more often it happens.

During the beachball i mentioned in the Activity Monitor app that the harddisk starts to read AND write something (about 20MB/s read and 20MB/s write), so the harddisk activity is HUGE and prevents PT to play back audio. Suddenly this behaviour stops and PT starts playing.

This happens regardless of the device i use in the Playback Engine (HDX, MBox Pro, Mac soundcard).

I tried simply everything from repair permissions to disabling network, every performance suggestion from Avid (spotlight off f.e.) is done...

Does anyone else on this planet have this problem?

[feliponk]

How is the size of the Disk Cache/Buffer Size when playing?

[clausiii]

I tried every size between "normal" and "13 GB". No difference.
btw, even 13GB would make Disk Cache senseless because it would just cache about 3-7% of the session.

[clausiii]

And buffer size: i tried every setting, but the buffer size doesn't make any sense on HDX systems, because it only takes effect when you use native plugins. And we don't use any.