Motu 828 Mkii Problems, screeching halts my recording Options

[Stradivarius]

Whenever I attempt to record or even just work with my files in Audiodesk, sooner or later my 828 unit will cease any playback and produce a piercing high-pitched feedback type sound until I turn it off.  It even persists to do it after I quit Audiodesk.  Sometimes it will even do it when the unit is on and Audiodesk isn't even open.   I am using a 1.6 G5 single processor and have run Disk Doctor and optimized it a number of times.  In fact, I am running Audiodesk from its own 120GB second hard drive with nothing else besides OSX.  I can't tell if problem starts because of playback, recording, or what have you--as it seems to do be able to start during anything.  I've restored factory defaults, installed all the latest downloads from MOTU.com and it just won't work.  We have used the unit on my friend's Powerbook G4 and it will sometimes do it on that machine, but it seems to be less often than on mine.  I also set it as my sound output in the System Preferences to see what would happen--and began to screech after not 6 minutes of iTunes. I need a solution to this extremely frustrating problem.  Please help me out.  Thanks.