Stalling graphics in logic 9.1.7 - colour apple wheel constantly shows up, slowing down work rate! Help.

Hi.

i've been playing around with my logic Audio settings trying to change the buffer size etc etc but i can't seem to confige Logic to stop stalling.

my problem is that i will be working on a project and then suddly, and very often, it will stall and the coloured wheel will come on but the audio loop i am working on will continue to play, then the image catches up once the wheel has stopped spinning.

this is most frustrating and slowing my work rate down substantially!

i have an apogee duet, and usuing my external WD FireWire drive for the audio drive while the system drive runs logic. (as I have for years!), i'm on the latest osx Lion too.

Model Name: Mac Pro

Model Identifier: MacPro1,1

Processor Name: Dual-Core Intel Xeon

Processor Speed: 2.66 GHz

Number of Processors: 2

Total Number of Cores: 4

L2 Cache (per Processor): 4 MB

Memory: 5 GB

Bus Speed: 1.33 GHz

at the end of my teather so some sort of idea's/suggestions would be much appreicated.

hope you can help, as this is same set up was working fine the last 2months ago before I upgraded to osc lion and the latest logic updated, i can believe I've tolerated it this long!!!!!!!

HELP!!!!