CPU error

[NuovoCello]

Hey there,

I have a question on a particular problem I have been experiencing.
First of all, I am currently using a Mac OS X version 10.5.8, dual 2.5 GHz PowerPC G5, with 6.5 DDR SDRAM. I use this computer for strictly recording purposes only, no internet. Most the songs I save on an external drive. The recording software I use is Pro Tools M-Powered version 8.1.0. And my audio interface is a M-Audio Pre USB Mobile.

I have run up to 16 midi tracks and 25 audio tracks on one session. Everything runs fine unless I exceed those numbers (usually).Then of course I see that pesky CPU overload error. The strange thing is, the only other time I see this error is with drum tracks. I may have one guitar track with a snare track, Hi-hat track, and bass track, and the error will occur. This only being four tracks at playback, and I get an error? Where as I stated previously, 25 audio tracks of all kinds of instruments except drums and it will not error until I exceed that. Does drums take up a lot of memory? I thought I had plenty of RAM. And I do not believe it is because I have to many plug-ins. I don't use much. I have tried changing the playback engine settings and buffer size, but it still doesn't work. I have also tried watching the system usage and the two things that climbs are CPU- usually only to about 18%, and the disk climbs to about 26% and then climbs rapidly to 90% when the error occurs.

Not sure what the deal is?

Much obliged and thank you,

-NC