CPU usage problems

[michaiel]

I have the following....
PT 8.01 / 003R
2009 Mac Pro Octo with 10.6
12 Gigs RAM
Session size (24 tracks) with approximately 3 RTAS Altiverb plugins (everything else is UAD Quad 2.0).

H/W Buffer Size - 128 Samples
Host Processors - 15
CPU Usage Limit - 95%
DAE Playback Buffer - (Defaults Level 2)

Recording midi drums at 128 Samples (H/W Buffer Size) to avoid SSD 3.5 Drums from having a noticeable delay while recording drums.

I continue to get CPU errors asking me to remove RTAS plugins. My PT monitor says I am running at 30% CPU.

Can anyone explain how I am running out of processing power on so little use of PT8?

Can anyone recommend better settings?
thanks