High drive activity under Mac OS 10.9.2 & Nuendo 6.5.4

I have experienced very high drive activity and memory usage at the same time using Nuendo 6.5.4, to the point where I thought it would drop out of record.

I switched to Nuendo 6.0.7 in the same studio, everything the same, and no problems.
I saw this message in console after high disk activity:

3/6/15 2:24:32.000 PM kernel[0] process Nuendo 6.5[278] thread 27211 caught burning CPU!; EXC_RESOURCE supressed due to audio playback

It has happened several times this week under 6.5.4, usually after a few hours… Disk activity goes crazy, and Acivity Monitor shows 7.99GB of 8GB memory Used.
Not happened at all under 6.0.7. Regular very low disc activity, Memory Used at a steady 3.48 GB.

Hardware is a MacMini, RME MADIface XT, External AVASTOR HDX-1500 1GB record drive (new). Recording 56 tracks @24b/48k.
Spotlight turned off for all drives.
This is for a system that is running 11 hours a day, and records for 6 (in 4x 1.5 hours sessions)

Any thoughts?